IPE-TM-600-11 — Magnetic Level Indicators

1. Purpose

This procedure provides design guidelines, to maintain a consistency of usage and design practice, for the selection of scale length, nozzle orientation, and nozzle spacing for magnetic level indicators.

2. Services Where Magnetic Level Indicators are Specified

Inflection Point Engineering’s standard policy is to use magnetic level indicators in only those services previously requiring OGV (Omit Gauge Valves) for the gauge glasses. These are typically caustic services. Magnetic level indicators are not required to be used on vessels/tanks that operate at atmospheric pressure. OGV gauge glasses may still be used.

If the customer desires to use magnetic level indicators in other services, Inflection Point Engineering would not object though each service will need to be evaluated. Magnetic level indicators can NOT be used in place of gauge glasses on steam drums. They can be used in addition to the gauge glasses.

Avoid using magnetic level indicators (especially “flag” type) where high accuracy in reading the level is required (e.g. using the level indicator to determine injection rates).

Each magnetic level indicator shall have a unique tag number (based on Instrument numbering system) associated with it on the P&ID.

3. Deletion of Stand-alone Level Transmitter

Inflection Point Engineering objects when a customer requests the deletion of the stand-alone level transmitter by adding a level transmitter to the magnetic level indicator. Explain Inflection Point Engineering policy which requires a “back-up” device, and do not change the P&IDs.

Inflection Point Engineering’s policy is to keep local and control center indication/control separate. The use of two magnetic levels (one with a transmitter, and one with local indication) is acceptable.

4. Information to Specify

Per Procedure the Contractor is to determine the number of magnetic level indicators, the measurement length, and the connection orientation.

All other required information is to be provided on Inflection Point Engineering Project Specification 612. Inflection Point Engineering design practice is for 1 inch flanged connections.

5. Nomenclature

ML = Measured length.

B = Length between the 100% indication and top of chamber.

A = Length between the 0% indication and bottom of chamber flange (normally 10 inches).

Note: Top-Bottom orientation is also available.

6. Specific Gravity Limitations

As the operating pressure increases, the minimum specific gravity that the float can sense increases due to float construction requirements.

Table 1

Minimum Specific Gravity

	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)	For Temperatures Up to 600°F (316°C)
PSIG	300	650	750	850	950	1050	1500	1700	2600	2800
bar (g)	20.6	44.8	51.7	58.6	65.4	72.3	103.4	117.2	179.2	193.0
No Change in dim A Minimum Specific Gravity	0.47	0.54	0.55	0.58	0.62	0.67	0.67	0.71	0.76	0.89
Absolute Minimum Specific Gravity	0.27	0.33	0.38	0.42	0.42	0.42	0.51	0.55	0.59	0.69

Table 1 shows the minimum liquid specific gravities for various operating pressures for vapor/liquid service only.

The “No Change in dim A” gravities are the minimum allowable with the standard dimension “A” of 10 inches.

The “Absolute Minimum Specific Gravities” are the absolute minimum, even with a change in dimension “A”.

The minimum specific gravity difference for liquid/liquid interface service is 0.10.

Nozzle spacing is the same for both vapor/liquid and liquid/liquid interface services.

The magnetic level indicator nozzle spacing, for side-side and side-bottom orientations, shall be in accordance with GR-13, “Magnetic Level Indicator Dimensions”. Note: This is normally a Contractor detail since Inflection Point Engineering standard practice is to use pipe columns and not specify a magnetic level indicator length, or nozzle orientation.

For those services that fall outside the above guidelines, consult with the Process Control Coordinator.

7. Nozzle Orientation

(Note: Nozzle orientation is normally a Contractor detail.) When side-side or side-bottom nozzle orientations are used, changes in operating specific gravity will not affect the nozzle spacing dimensions for the range of operating pressures/gravities shown in Table 1 (Minimum Specific Gravity).

7.1 Side-Side

Operating specific gravities that would require a change in dimension “A” and/or “B” will not change the nozzle spacing dimensions.

7.2 Side-Bottom

a. Operating specific gravities that would require a change in dimension “B” will not change the nozzle spacing dimensions.

b. Operating specific gravities that would require a change in dimension “A” will not normally change the nozzle spacing dimensions, if the same dimensional allowance (11 inches) used by Inflection Point Engineering for the bottom flange/elbow/nipple assembly (bottom connection) is used for the magnetic level indicator bottom connection.

c. Based upon the minimum nozzle spacing for external displacement type level instruments (Procedure IPE-TM-300-02), there are 11 inches allowed for the bottom flange/elbow/nipple assembly. The minimum dimension required for the fittings ranges from 3 3/16 inches (1" Class 150) to 5 ¼ inches (2" Class 1500). Using 1 inch for the bottom visual overlap, this leaves an overage of 6 3/16 inches (1" Class 150) to 4 ¾ inches (Class 1500) available for changes in dimension “A”.

d. Based on K-TEK factory information, an increase in dimension “A” will be required for specific gravities less than those indicated in Table 1. The required increase in dimension “A” is calculated by K-TEK for each operating case. However, K-TEK indicated that 20 inches would be an estimated maximum for any low specific gravity service. This would be a 10 inch increase over the normal 10 inch requirement. Address special situations as they occur.

Additional Design Recommendations

Note: These are normally Contractor details. Where Inflection Point Engineering specifically requires free draining (bottom connected) it shall be indicated on the P&ID.

Magnetic Level Indicators