Vessel Level Instrument and Level Indicator Installation

1. Purpose

This procedure describes the Inflection Point Engineering practice for the specification of liquid level detection and indication instruments on vessels. In this procedure, the level detection instruments (level transmitters) are either displacement type or differential pressure type instruments. Visual level indicators are either gauge glasses or magnetic indicators.

2. Contents

1. Purpose 1

2. Contents 1

3. Level Control Philosophy 2

3.1 Automatic Level Control 2

3.2 Automatic Level Control - Low Accumulation Rate 2

3.3 Manual Level Control 2

3.4 Batch Filling and Withdrawal 2

3.5 Level Instrument Alarms and Shutdowns 2

4. Level Instrument Selection 4

4.1 Level Transmitters 5

4.2 Visual Level Indicators 7

4.3 Level Instrument, Nozzle, and Piping Sizes 8

4.4 Level Instrument Connection Orientations (End or Side Connections) 9

5. Standard Practice 10

5.1 General Requirements 10

5.2 Pipe Column Details 11

5.3 Bridle Installation 11

5.4 Direct Connected Level Instruments 11

5.5 Block Valves 11

5.6 Gauge Valves 12

5.7 Piping and Instrument Diagram Representation 12

6. Location of Nozzles on Vessels 12

6.1 Nozzle A - Vertical Vessel; Lower Nozzle Location 13

6.2 Nozzle B - Vertical Vessel; Upper Nozzle Location 13

6.3 Nozzle C - Drop Leg on Horizontal Vessel; Upper Nozzle Location 14

6.4 Nozzle C - C - Horizontal Vessel; Lower Nozzle Location 14

6.5 Nozzle D - Horizontal Vessel; Upper Nozzle Location (normal) 15

6.6 Nozzle E - Horizontal Vessel; Upper Nozzle Location (alternate) 15

7. Normal Liquid Level Locations 15

7.1 Vertical Vessels 16

7.2 Horizontal Vessels 16

7.3 Drop Legs 16

8. Details for Drop Leg Interface Level Instruments 16

9. References 17

Attachment 1 Spacing for Displacement Type Level Transmitter Connections for Class 150, 300, and 600 Flanges 19

Attachment 2 Spacing for Displacement Type Level Transmitter Connections for Class 900, 1500 & 2500 Flanges 20

Attachment 3 P&I Diagram Representation for Vertical Vessels 21

Attachment 4 P&I Diagram Representation for Horizontal Vessels and Drop Legs 22

3. Level Control Philosophy

3.1 Automatic Level Control

Liquid levels are generally maintained with a continuously operating control valve in either the liquid inlet or the liquid outlet line. In such cases the level transmitter range is generally determined by liquid residence time requirements.

3.2 Automatic Level Control - Low Accumulation Rate

If the accumulation rate is relatively low, the level transmitter may be used to control the stroke volume of a positive displacement pump or it may open and close a small on-off control valve. In such cases the level transmitter range is determined by flow and time constraints.

3.3 Manual Level Control

In systems with intermittent or very low liquid accumulation rates, manual operation of a valve in the outlet line may be considered. Instrumentation may consist of a visual level indicator alone or a level transmitter with a visual level indicator.

Manual level control is usually only employed if the operation frequency is no greater than once per eight hour shift. In such installations, it is Inflection Point Engineering practice to provide sufficient vessel volume within the level instrument range to contain ten hours or more of liquid accumulation.

When specifying manual level control, the visual level indicator must provide visible indication as low as possible within the limitations of the lower level nozzle locations. In manually drained systems add the note “GAUGE GLASS MUST BE READABLE FROM VALVE” to the Inflection Point Engineering P&ID's.

Batch Filling and Withdrawal

Special considerations are required for manual batch filling and withdrawal operation such as Caustic Prewash, Caustic Scrubbers, etc. Such applications shall have gauge glass sizes and locations specified on the Inflection Point Engineering P&ID's.

3.5 Level Instrument Alarms and Shutdowns

Evaluate each level instrument installation to determine high and/or low level alarm/shutdown requirements as follows:

a. Most installations in which the liquid is pumped shall have low level alarms.

b. Installations with manual level control shall have high level alarms.

c. Trayed vessels shall have high level alarms at the bottom of the column.

d. Drop leg interface level transmitters shall have high level alarms. Automatic draw-off services shall have high and low level alarms.

e. Vessels in compressor suction service shall have high level alarms and shutdowns when the suction vessel is expected to normally contain liquid, or when gas entering the suction vessel could be a dew point vapor.  PSA hydrogen is excluded from this requirement. The level instrumentation must be configured in a 2 out of 3 logic to minimize false trips and insure a reliable system.  Consult the Technology Specialist and Process Control Coordinator to determine if the suction drum instrumentation requires a SIL.  The Design Engineer is accountable to insure the proper system is used for the application.

f. Level shutdowns generally use 14" displacement type level transmitters (not float type level switches), with each shutdown transmitter (SDT) having separate connections on the vessel.

Low Liquid Level (LLL) trip point:

The trip / COR will need to be determined for low level applications (such as pump shutdown on low vessel level) since the COR cannot be less than 13" (330) above the vessel bottom tangent line. The connection orientation for the shutdown transmitter is typically the top / side design (see sketches at left).

For ease of engineering, the trip set point will generally be the COR of the SDT (i.e. 50% of the range), and the critical LAL will be set at 60%.

In all cases, the Process Control Coordinator shall confirm the level instrument placement.

High Liquid Level (HLL) trip point:

The COR of the SDT is at the 100% point of the control/indicating transmitter for a high level shutdown. The connection orientation for the shutdown transmitter is typically the side/bottom design (see sketch below). The side/bottom design ensures that the chamber is free draining, and satisfies those units (such as HF) that require the chamber to be completely drained. NOTE: Side/bottom will generally NOT fit when the control/indicating level is a 14”. If that is the case, and free draining is not a requirement, then another orientation can be used.

For ease of engineering, the trip set point will be the COR of the SDT (i.e. 50% of the range), and the critical LAH will be set at 40%.

There may be those applications (such as vaporizers) where the trip / COR may not follow the above guidelines.

In all cases the Process Control Coordinator shall confirm the level placement.

g. The vertical elevation of the vessel nozzles required for high and low level shutdown instruments should be based on the location of the COR of the shutdown instrument as detailed in the previous section, as well as the space required. The vessel specification should show the nozzles for the level shutdown instrument at an elevation separate from the level control instrument. The contractor will be responsible for determining the piping arrangement used to connect the shutdown instrument to the vessel nozzles.

The P&ID should graphically show the shutdown level nozzle connections separate from the level control nozzles. Nozzle elevations dimensions should not be shown on the P&ID, as this duplicates information already contained in the 301 vessel specification. The COR of the shutdown instrument should be indicated on the P&ID, as described in the Section f above.

4. Level Instrument Selection

Liquid level instrumentation typically consists of a level transmitter and one or more visual level indicators, though some manual services do not require the use of a transmitter. In extremely dirty or fouling services use a second level transmitter in place of a visual level indicator. Determine the type of level instrument using the following guidelines:

4.1 Level Transmitters

Select level transmitters on the basis of process requirements and transmitter range. Transmitters typically specified by Inflection Point Engineering are described below:

a. Displacement Type Level Transmitters

Displacement type level transmitters sense liquid level via an internal displacer and are specified in clean hydrocarbon liquid, water, or liquid/liquid interface services. In displacement type level transmitters the displacer weight (buoyancy force) is sensed as the liquid level rises and falls.

For flange classes 150, 300, and 600 displacer lengths range from 14" (360 mm) to 120" (3050 mm). The maximum length normally specified by Inflection Point Engineering is 60" (1520 mm). Attachment 1 lists the standard ranges and nozzle spacing.

For flange classes 900, 1500, and 2500 displacer lengths are 14" (360 mm) and 32" (810 mm). Attachment 2 lists the standard ranges and nozzle spacing.

b. Differential Pressure Type Level Transmitters

Differential pressure type level transmitters (sometimes called DP cells) sense liquid level via the differential pressure between the top and bottom level taps. They can be used in nearly all level services, though they are not frequently used in liquid interface services.

Differential pressure type level transmitter ranges begin at 14" (350 mm) as per the following table. The minimum differential pressure instrument specified by Inflection Point Engineering is typically >60" (1520). Inflection Point Engineering generally avoids their use below 60" (1520 mm).

inches	mm(Note 1)
14	350
32	800
48	1200
60	1500
72	1800
84	2100
96	2400
108	2700
120	3000
Note 2	Note 2

Note 1: Metric dimensions are standard nominal sizes; not exact conversions from standard English dimensions.

Note 2: For differential pressure type level transmitter ranges longer than 120" (3000 mm), round off the calculated range to the nearest foot (English) or to the nearest 100 mm (metric). Provide the dimensions on the P&ID in units of inches or millimeters.

Differential pressure type level transmitters with or without remote seals (capillary type) below 32" (800 mm) range shall not be used without consulting the Process Control Coordinator.

Differential pressure type level transmitters no longer require separate connections on the vessel; level connections are made on the pipe column. On vessels with multiple overlapping pipe columns, the lower connection is made on the bottom pipe column and the upper level connection is made on the top pipe column.

When the customer requires direct connections to the vessel, the distance between the upper and lower connection is exactly the transmitter range.

c. Guided Wave Radar Type Level Transmitters

Guided wave radar type level transmitters sense liquid level using Time Domain Reflectometry (TDR). TDR basically uses an ultra high speed timing circuit to measure when an electromagnetic energy pulse traveling down the probe is reflected from a surface that has a higher dielectric than the fluid the pulse initially started in. Guided wave radar can generally be used with liquids that have a dielectric between 1.4 and 100.

There are issues with lighter hydrocarbons, especially at elevated temperature as the dielectric can be too low. Interface levels can also be problematic. Guided wave radar should not be used without first consulting the Process Control Coordinator.

Ranges can follow those for the differential pressure type levels, but should normally be limited to 60" (1500 mm). The maximum is 180" (4600 mm).

d. Float Type Level Switches

Float type level switches have an internal float that triggers an electrical switch as the liquid level rises above or falls below a fixed point. Level switches are generally not specified by Inflection Point Engineering.

When required, specify level switches in accordance with Procedure IPE-TM-600-01, “Nozzle Spacing for Float Actuated Level Switches”.

4.2 Visual Level Indicators

Select the type of visual level indicator on the basis of process requirements. The most common types of visual level indicators are described below:

a. Gauge Glasses - Reflex Type

Reflex type level gauges have a single vision slot in which light can enter the gauge chamber to determine the liquid level. Above the liquid level, glass prisms reflect the surrounding light. Below the liquid level, the liquid fills the prisms causing the glass to become relatively transparent compared to the glass above the liquid level. Use reflex type gauges (P&ID designations LG-R and LG-RLT) in most clean, single liquid phase, process water, and low temperature (LT) services.

b. Gauge Glasses - Through-View Type

Through-view type level gauges have vision slots on both sides of the gauge chamber. Light enters the gauge from the side opposite the observer so that both the liquid level and its characteristics can be seen. Use through-view type level gauges (P&ID designations LG-T, LG-TG, LG-TK, and LG-TLT) in dirty, liquid interface, caustic, acid, boiler feedwater and steam condensate, and low temperature (LT) services. Specify special gasket materials such as Grafoil, designated (G), or special glass shielding materials such as Kel F, designated (K) for specific services.

c. Gauge Glasses - Tubular Type

Tubular type level gauges have a transparent tube through which the observer can see the liquid level and the liquid’s visual characteristics. Tubular type level gauges (P&ID designation LG-B) are rarely used and only in clean, single phase, and atmospheric pressure services.

d. Magnetic Level Indicators

Magnetic level indicators have a non-magnetic chamber with an internal float containing magnets. The float responds to variations in the liquid level. An indicating device mounted outside the chamber is magnetically coupled to the float. Magnetic level indicators (P&ID designation LG-M) are adaptable to a wide range of process requirements for level and interface services.

4.3 Level Instrument, Nozzle, and Piping Sizes

Inflection Point Engineering standard practice for level instrument size limitations, piping sizes, and minimum nozzle requirements is indicated in the following table:

	Displacement Type Transmitter	Guided Wave Radar (Note 1)	Differential Pressure Type Transmitter	Gauge Glasses	Magnetic Level Indicators
	14" (360 mm)	14" (350 mm)	60" (1520 mm)	1'-0" (300 mm)	16" (410 mm)
	60" * (1520 mm)	180" (4600 mm) (Note 1)	no practical limit	5'-6" ** (1650 mm)	15'-0" (4600 mm)
Piping Size	1-1/2"	1-1/2"	3/4"	3/4"	1"
Minimum Nozzle Size	1-1/2" ***	1-1/2" ***	1" ***	1" ***	1" ***

* 60" (1520 mm) is the largest size normally specified by Inflection Point Engineering. Inflection Point Engineering will specify up to 120" (3050 mm) if requested by the customer.

** For level ranges over 5'-6" (1650 mm) two or more overlapping gauge glasses are required.

***For lined vessels with direct connected level instruments, the minimum nozzle size is 2".

Note 1 Guided Wave Radar ranges can generally be made between 6" and 900" (150 and 22860 mm). Selected Ranges should generally use the same ranges as the Differential Pressure Type, up to 180" (4600 mm) maximum. The maximum is due to pipe column/ chamber length restrictions.

4.4 Level Instrument Connection Orientations (End or Side Connections)

a. Displacement type level transmitters, gauge glasses, and magnetic level indicators are available with connection orientations defined below.

** Note: Guided Wave Radar levels do not allow for the Top connection in the table as the top connection is where the probe/transmitter is installed. All other aspects of the connections are the same as Displacement Type level instruments (including spacing dimensions listed in Attachments 1 and 2).

CONNECTION ORIENTATION	DESCRIPTION
Top / Bottom	The upper connection is top mounted and the lower connection is bottom mounted.
Top / Side	The upper connection is top mounted and the lower connection is side mounted.
Side / Bottom	The upper connection is side mounted and the lower connection is bottom mounted.
Side / Side	Both upper and lower connections are side mounted.

b. Specify one of the above connection orientations for displacement type level transmitters.

c. Gauge glasses and magnetic level indicators installed on pipe columns are indicated on Inflection Point Engineering P&ID's as top / bottom mounted. The contractor makes the final selection based on the requirements of Inflection Point Engineering Standard Drawing 8-121.

d. Specify the connection orientation only where needed, applying the following guidelines:

(1) Typically use the top / side connection orientation at the bottom of vertical vessels for displacement type level transmitters.

(2) On horizontal vessels use the top / side orientation for displacement type level transmitters unless there is sufficient space available for the top / bottom orientation.

(3) On drop legs use the side / bottom orientation for displacement type level transmitters.

(4) Top / bottom or side / bottom orientations are required in HF acid services.

(5) The side / side orientation is rarely used because it is difficult to “fit-up”. It is only used when customer requests or when other orientations do not “fit”.

(6) Connection orientations do not apply to differential pressure type level transmitters.

(7) Orientations and dimensions for displacement type level transmitters are provided in Attachments 1 and 2.

(8) Orientations and dimensions for gauge glasses and magnetic level indicators are provided in General Reference GR-12, “Level Gauge Dimensions”, and GR-13, “Magnetic Level Indicator Dimensions”.

5. Standard Practice

5.1 General Requirements

Inflection Point Engineering standard practice requires liquid level instruments to be installed on a 2" diameter pipe column per Inflection Point Engineering Standard Drawing 8-121 except on small vessels where pipe columns are not practical. The alternate piping configuration (sometimes referred to as a “bridle”) indicated on Standard Drawing 8-121 may be applied by the contractor if requested by the customer.

Some customers require level instruments to be directly connected to the vessel without the use of pipe columns. Also, single level indicators without level transmitters are generally installed directly on the vessel.

Level instruments, pipe columns, and bridles are not connected to flowing lines. The flowing liquid has an adverse hydraulic effect on the liquid column in the level instrument resulting in false level readings.

Inflection Point Engineering Standard Drawing 8-121 instructs the contractor to provide one or more visual level indicators to cover, as a minimum, the entire range of the specified level transmitter. The entire liquid portion of the vessel is not covered unless specifically required by the customer. Standard Drawing 8-121 also instructs the contractor that if more than one visual level indicator is required, they must provide a 1" (25 mm) minimum visual overlap between each indicator.

5.2 Pipe Column Details

Inflection Point Engineering specifies a 2" pipe column connected to the vessel with two 2" nozzles. Flanges are ASME Class 300 minimum*, or higher, if required by the mechanical design conditions and metallurgy of the vessel.

*Class 300 flanges with 8 flange bolts are specified in place of Class 150 flanges with 4 flange bolts as they are more tolerant of pipe stresses and therefore are less prone to leakage.

The maximum pipe column length is 15'-0" (4600 mm). DP level instrumentation ranges beyond 15'-0" (4600 mm) require the use of two or more overlapping pipe columns. The overlap between pipe columns must be sufficient to provide a 1" minimum visual overlap between adjacent level indicators.

5.3 Bridle Installation

Vessel nozzles for bridles are 2", Class 300, consistent with pipe column nozzles. The maximum distance between bridle level nozzles is 9'-0" (2700 mm). Pipe columns are required for level nozzle spacing greater than 9'-0" (2700 mm).

5.4 Direct Connected Level Instruments

When the customer selects the direct connected option, Inflection Point Engineering will specify the type, range, and connection orientation of the level transmitter and the size and flange class of the connecting vessel nozzles. Inflection Point Engineering will also specify the type of visual indicator and the size and flange class of the connecting vessel nozzles. The number of visual indicators required to cover the level transmitter range and the elevations of the connecting vessel nozzles are the responsibility of the contractor.

5.5 Block Valves

Inflection Point Engineering does not specify isolation block valves on pipe columns or bridles. Inflection Point Engineering does specify isolation block valves for each individual level transmitter and level indicator. The block valves added to the pipe column offer a potential for an operator to close a single block valve and disable multiple instruments.

Vessel isolation block valves on a pipe column (or bridle) should only be added upon customer request. These isolation block valves shall include a note on the P&ID that they should be “Locked Open.” The pressure instruments may be separated from the pipe column by moving the pressure transmitter and pressure gauge to a dedicated nozzle on the vessel, however this is not a requirement as long as the new isolation valves are locked open. If the customer does not accept keeping the isolation valves as locked open then dedicated nozzles should be provided for the pressure instruments and level transmitter.

The customer should be informed of Inflection Point Engineering’s piping arrangement when block valves are to be provided on pipe columns. The customer may then elect to provide separate connections for all instruments to avoid single point failures. This is acceptable, as avoiding a single point failure is the intent of Inflection Point Engineering’s design.

5.6 Gauge Valves

Gauge glasses are normally specified with ball-check gauge valves. The ball-check instantly closes in case of glass breakage. The ball-check valve can also be closed manually.

In some services ball-check gauge valves are omitted because of potential fouling. Use the P&ID designation OGV (omit gauge valve) to indicate omission of the gauge valve.

5.7 Piping and Instrument Diagram Representation

Typical P&I Diagram representations for vertical vessel level instruments are provided on Attachment 3. Representations for horizontal vessels and drop legs are provided on Attachment 4.

6. Location of Nozzles on Vessels

The location of level instrument connections must also take into consideration certain restrictions relating to the vessel construction. The following sketches indicate typical locations for level instrument connections on vessels.

Note: Never locate level instrument connections on the bottom of a vessel because of potential plugging.

The following sketches identify nozzle locations in conjunction with Sections 6.1 through 6.6.

6.1 Nozzle A - Vertical Vessel; Lower Nozzle Location

As indicated in the sketch at the right, the centerline of the nozzle at this location on a vertical shell is located 6" (150 mm) from the tangent line in order to provide for the straight flange length of the elliptical head or pipe cap, one-half of the nozzle outside diameter, and clearance between the nozzle-to-shell weld and the head-to-shell butt weld.

Good design and fabrication practice avoids interference of welds whenever possible. The 6" (150 mm) dimension normally provides clearance for connections up to 2" in diameter. It does not allow for a reinforcing pad since these small nozzles seldom require additional shell reinforcement to replace the removed metal area. WIN254 will calculate the dimension if a nozzle size larger than 2" is applied (3" nozzles typically require a 8” (200 mm dimension). The larger dimension will be added to the vessel tangent length and other dimensions related to the tangent line.

6.2 Nozzle B - Vertical Vessel; Upper Nozzle Location

• On vertical vessels without internals or vapor - liquid disengagement (e.g., surge drums) locate the upper level instrument nozzle a minimum of 6" (150 mm) below the vessel’s top tangent line. This provides the maximum level transmitter size to take advantage of the entire vessel’s volume for surge.

The 6" (150 mm) dimension normally provides clearance for connections up to 2" in diameter. It does not allow for a reinforcing pad since these small nozzles seldom require additional shell reinforcement to replace the removed metal area. WIN254 will calculate the dimension if a nozzle size larger than 2" is applied (3" nozzles typically require a 8" (200 mm dimension). The larger dimension will be added to the vessel tangent length and other dimensions related to the tangent line.

b. On fractionators typically locate the upper level instrument nozzle a minimum of 12" (300 mm) below the bottom tray to provide adequate clearance between the tray and the protective baffle for the nozzle. This location may be moved by the Contractor to a lower position provided the required level transmitter and level indicator(s) can be accommodated.

c. On other vertical vessels locate the upper level instrument nozzle a calculated (minimum) dimension below mesh blankets, inlet nozzles, vapor - liquid disengaging space, etc. This location may be moved by the Contractor to a lower position provided the required level transmitter and level indicator(s) can be accommodated.

• Whenever the upper level nozzle is in range of an incoming stream, the nozzle must be protected with a baffle to prevent impingement and resultant false level indications. Specify baffles per Inflection Point Engineering Standard Drawing 3-115, Type I.

• When a vertical vessel is liquid full and has a liquid – liquid interface, locate the upper level instrument nozzle on the side of the vessel. Never locate it on the top of the vessel.

6.3 Nozzle C - Drop Leg on Horizontal Vessel; Upper Nozzle Location

Locate the drop leg upper level instrument nozzle a minimum of 8" (200 mm) above the bottom of the horizontal shell. This position has the advantage of being above the level of the horizontal vessel liquid outlet riser [normally 6" (150 mm) above the bottom of the vessel] and below the minimum controlled liquid level in the horizontal vessel, thereby maintaining constant contact with the lighter liquid.

6.4 Nozzle C - C - Horizontal Vessel; Lower Nozzle Location

Typically locate the lower level instrument nozzle a maximum of 0.45D below the centerline of the vessel to provide maximum flexibility for level instrument installation. This location may be raised to a higher position by the contractor to allow for connection requirements. Placement of the nozzle lower than 0.45D below the vessel centerline is a mechanically difficult fabrication (see sketch below) that may be prone to plugging. 3” nozzles should be located 0.45D - ½" (0.45D - 15 mm) below the vessel centerline.

Alternately locate the lower level instrument nozzle at the top of the riser on the vessel liquid outlet nozzle, a maximum dimension of “H” below the centerline of the vessel (see sketch below). This location of the nozzle may be raised by the contractor to allow for connection requirements.

6.5 Nozzle D - Horizontal Vessel; Upper Nozzle Location (normal)

Locate the upper level instrument nozzle on the top of the vessel. This position ensures sufficient available vertical distance to install the required level instrumentation. Do not use this nozzle location if the vessel is liquid full with a liquid – liquid interface. Use nozzle E instead.

6.6 Nozzle E - Horizontal Vessel; Upper Nozzle Location (alternate)

Due to customer request, or platform arrangement it may be necessary for the contractor to locate the upper level instrument nozzle near the top of the horizontal shell. Indicate that this vessel nozzle may be located on the top or the upper side of the vessel. The upper side location of the nozzle may require the use of the side / side orientation for the level transmitters and level indicators. Use this nozzle location for liquid full vessel with a liquid-liquid interface.

Note: Specific technologies may have special requirements; refer to technology specialists for guidelines.

7. Normal Liquid Level Locations

Use the liquid residence times found in Procedure IPE-TM-300-03, “Vessel Nozzles, Internals, and Ancillaries” for vertical vessels and in Procedure IPE-TM-300-04, “Vessel Sizing Guidelines - Receivers, Separators and Drums” to calculate the range of level transmitters. Alternately, customer instructions in the Basic Engineering Design Questionnaire (BEDQ) may take precedence. In a very few exceptions, criteria other than liquid residence time may determine level transmitter ranges.

Note: Specific technologies may have special requirements; refer to technology specialists for guidelines.

The term center of range (COR) is used to designate the center point of a level instrument. The term normal liquid level is used to designate the normal liquid level which may be at a different point above or below the COR.

7.1 Vertical Vessels

In most vertical vessels the minimum liquid level is located at the centerline of the lower level nozzle. For a 2" nozzle the lower level nozzle is 6" (150 mm) above the vessel bottom tangent line. The center of range (COR) is 6" (150 mm) plus half the level transmitter range above the vessel bottom tangent line. The maximum liquid level is 6" (150 mm) plus the full level transmitter range. Bottom connected displacement type level transmitters are not normally used in vertical vessels.

7.2 Horizontal Vessels

Typically set the normal liquid level for horizontal vessels at the centerline of the vessel. Consider setting the normal liquid level higher than the vessel centerline if the cross-sectional area of the vessel required for vapors is small compared to the area required for liquid. Consider setting the normal liquid level lower than the vessel centerline if the cross-sectional area of the vessel required for vapor is large compared to the area required for liquid. Follow the guidelines contained in Procedure IPE-TM-300-04 striving to minimize the vessel diameter.

Different criteria may be employed for horizontal vessels in liquid/liquid services.

7.3 Drop Legs

In the standard drop leg design, 3'-6" (1100 mm) long, the liquid interface level elevation should provide sufficient time for disengagement of the light liquid from the heavy liquid. The cross-sectional area of the drop leg must be great enough to ensure that the downward velocity of the heavy liquid is less than the disengaging velocity of the light liquid.

Typically locate the maximum interface level flush with the bottom of the horizontal vessel at the top of the drop leg. When using a 14" (360 mm) displacement type level transmitter, locate the normal interface level (COR) 7" (180 mm) below this point. When using a 32" (810 mm) displacement type level transmitter, locate the normal interface level 16" (405 mm) below the bottom of the vessel and the side / side connection orientation is required. The 32" (810 mm) level transmitter is rarely used, only when specific process requirements dictate.

8. Details for Drop Leg Interface Level Instruments

Drop legs (sometimes called “boots”) are added to horizontal vessels when the system contains two liquid phases and the volume of the heavy liquid phase is much smaller than the volume of the lighter liquid phase. Size drop legs in accordance with Procedure IPE-TM-300-04. As indicated in IPE-TM-300-04, the minimum drop leg length is 3'-6" (1100 mm) which will satisfy most drop leg requirements, except services in which large volumes of wash water are separated from the hydrocarbon liquid. In situations where the volume of the heavy phase is close to the volume of the lighter phase, consultation with the appropriate technology specialist is required.

For most applications in which the 3'-6" (1100 mm) drop leg length is satisfactory, the following interface level instrument arrangement with a 14" (360 mm) displacement type transmitter and a gauge glass is typical. The contractor shall determine the gauge glass size according to Standard Drawing 8-121.

ENGLISH DIMENSIONS

Dimensional Arrangement P&I Representation

METRIC DIMENSIONS

Dimensional Arrangement P&I Representation

For automatic draw-off and manual draining applications, specify a 14" (360 mm) displacement type level transmitter with a side / bottom connection orientation installed with the center of range (COR) 7" (180 mm) down from the bottom of the horizontal vessel.

9. References

Vessel Nozzles, Internals and Ancillaries

Vessel Sizing Guidelines - Receivers, Separators and Drums

Nozzle Spacing for Float Actuated Level Switches

Gauge Glasses

Baffles

External Displacement Level Instrument Piping Assemblies

External Displacement Level Instrument Piping Assemblies

HF Acid Service

Gauge Glass Piping Assemblies General Service

Typical Level Instrument Installation

Gauge Glass Piping Assemblies Caustic or Other Service

Not Requiring Gauge Valves

Gauge Glass Piping Assembly HF Acid Service

Level Gauge Dimensions

Magnetic Level Indicator Dimensions

Attachment 1 Spacing for Displacement Type Level Transmitter Connections for Class 150, 300, and 600 Flanges

CONNECTION ORIENTATION	RANGE	RANGE	14"	32"	48"	48"	60"	60"	72" *	84" *	96" *	120" *
Top / Bottom		A B C	4'-0" 2'-2" 1'-10"	5'-6" 2'-11" 2'-7"	5'-6" 2'-11" 2'-7"	6'-10" 3'-7" 3'-3"	6'-10" 3'-7" 3'-3"	7'-10" 4'-1" 3'-9"	8'-10" 4'-7" 4'-3"	9'-10" 5'-1" 4'-9"	10'-10" 5'-7" 5'-3"	12'-10" 6'-7" 6'-3"	12'-10" 6'-7" 6'-3"
Top / Side		A B C	2'-9" 2'-2" 7"	4'-3" 2'-11" 1'-4"	4'-3" 2'-11" 1'-4"	5'-7" 3'-7" 2'-0"	5'-7" 3'-7" 2'-0"	6'-7" 4'-1" 2'-6"	7'-7" 4'-7" 3'-0"	8'-7" 5'-1" 3'-6"	9'-7" 5'-7" 4'-0"	11'-7" 6'-7" 5'-0"	11'-7" 6'-7" 5'-0"
Side / Bottom		A B C	2'-5" 7" 1'-10"	3'-11" 1'-4" 2'-7"	3'-11" 1'-4" 2'-7"	5'-3" 2'-0" 3'-3"	5'-3" 2'-0" 3'-3"	6'-3" 2'-6" 3'-9"	7'-3" 3'-0" 4'-3"	8'-3" 3'-6" 4'-9"	9'-3" 4'-0" 5'-3"	11'-3" 5'-0" 6'-3"	11'-3" 5'-0" 6'-3"
Side / Side		A B C	1'-2" 7" 7"	2'-8" 1'-4" 1'-4"	2'-8" 1'-4" 1'-4"	4'-0" 2'-0" 2'-0"	4'-0" 2'-0" 2'-0"	5'-0" 2'-6" 2'-6"	6'-0" 3'-0" 3'-0"	7'-0" 3'-6" 3'-6"	8'-0" 4'-0" 4'-0"	10'-0" 5'-0" 5'-0"	10'-0" 5'-0" 5'-0"

CONNECTION ORIENTATION	RANGE, mm	RANGE, mm	360	810	1220	1520	1830 *	2130 *	2440 *	3050 *
Top / Bottom		A B C	1220 660 560	1675 890 785	2085 1095 990	2390 1245 1145	2695 1400 1295	3000 1550 1450	3300 1700 1600	3910 2005 1905
Top / Side		A B C	840 660 180	1295 890 405	1705 1095 610	2005 1245 760	2315 1400 915	2615 1550 1065	2920 1700 1220	3530 2005 1525
Side / Bottom		A B C	740 180 560	1190 405 785	1600 610 990	1905 760 1145	2210 915 1295	2515 1065 1450	2820 1220 1600	3430 1525 1905
Side / Side		A B C	360 180 180	810 405 405	1220 610 610	1520 760 760	1830 915 915	2130 1065 1065	2440 1220 1220	3050 1525 1525

* Used only when specifically requested by the Customer.

Attachment 2 Spacing for Displacement Type Level Transmitter Connections for Class 900, 1500 & 2500 Flanges

			Class 900 & 1500	Class 900 & 1500	Class 2500	Class 2500
CONNECTION ORIENTATION	RANGE	RANGE	14"	32"	14"	32"
Top / Bottom		A B C	4'-8" 2'-5" 2'-3"	6'-2" 3'-2" 3'-0"	5'-0" 2'-7" 2'-5"	6'-6" 3'-4" 3'-2"
Top / Side		A B C	3'-0" 2'-5" 7"	4'-6" 3'-2" 1'-4"	3'-2" 2'-7" 7"	4'-8" 3'-4" 1'-4"
Side / Bottom		A B C	2'-10" 7" 2'-3"	4'-4" 1'-4" 3'-0"	3'-0" 7" 2'-5"	4'-6" 1'-4" 3'-2"
Side / Side		A B C	1'-2" 7" 7"	2'-8" 1'-4" 1'-4"	1'-2" 7" 7"	2'-8" 1'-4" 1'-4"

			Class 900 & 1500	Class 900 & 1500	Class 2500	Class 2500
CONNECTION ORIENTATION	RANGE, mm	RANGE, mm	360	810	360	810
Top / Bottom		A B C	1420 735 685	1880 965 915	1525 790 735	1980 1015 965
Top / Side		A B C	915 735 180	1370 965 405	970 790 180	1420 1015 405
Side / Bottom		A B C	865 180 685	1320 405 915	915 180 735	1370 405 965
Side / Side		A B C	360 180 180	810 405 405	360 180 180	810 405 405

Attachment 3 P&I Diagram Representation for Vertical Vessels

Attachment 4 P&I Diagram Representation for Horizontal Vessels and Drop Legs