Sealing Requirements for Centrifugal Pumps

1. Purpose

This procedure describes the details to be shown in project specifications and on Piping & Instrument Diagrams (P&ID’s) when sealing plans are specified for centrifugal pumps.

2. General

Sealing requirements for centrifugal pumps are described in Procedure ". This information provides the Design Engineer with background information on the type of sealing currently specified for centrifugal pumps and provides guidelines on where to find this information in Project Specification 501, Centrifugal Pumps and Drivers. This is sufficient to give an understanding of why certain systems are specified, and when modifications to these systems may be considered. Some examples of details of sealing requirements for certain process applications are given in Attachment 1. For examples of sealing details for the Phenol Process Unit see Instructions , ", and ".

Contact the Pump Specialist to answer questions about the type of sealing required for a particular service. The pump specialist has mechanical seal material of construction chemical compatibility charts and temperature limits for both metal components and elastomeric parts.

Reference API Standard 610, “Centrifugal Pumps for Refinery Services” and API Standard 682, “Shaft Sealing Systems for Centrifugal and Rotary Pumps” as necessary.

3. Description of Sealing Requirements In Project Specification 501

See Project Specification 501, Seal Data and Seal Flush Piping for sealing requirements for each pump.

4. Seal Data Section

Specify the seal type for the pump in the Mechanical Seal Data section of Project Specification 501. Specify mechanical seals as either single, single with auxiliary sealing device, pressurized dual, unpressurized dual or non-contacting pressurized dual. Bellows seals are also available for single, unpressurized dual and pressurized dual type seals. See Section 5. If unpressurized dual mechanical seals are specified, always specify an auxiliary flush plan. If pressurized mechanical seals are specified, use an auxiliary flush plan or external flush fluid. In all cases, include a detail on the P&ID. When specifying a seal flush or auxiliary seal flush from an external source, specify the fluid properties (supply temperature, specific gravity, specific heat, vapor pressure) in the project specification.

5. Bellows Seals

Bellows type seals are available for single, unpressurized dual or pressurized dual type seals. Specify bellows seals for high temperature services, over 550ºF (288ºC). Bellows seals are not suitable for high pressures. When the normal suction pressure is over 450 psig do not specify bellows seals. The most likely place for a high pressure, high temperature seal is the Platforming steam generation section circulating water pump. For this service specify a single balanced cartridge.

6. Seal and Auxiliary Seal Flush Piping Plans

Seal flush Piping Plans refer to flushing of the inner or process side seal face of the primary seal. Auxiliary Seal Flush Piping Plans refer to flushing outside the primary seal face or between dual seals.

6.1 Seal Flush Piping Plans

Seal Flush Piping Plan piping details are identified by a number found in API 610. These plans show standard piping requirements for injecting seal flush fluid to the high-pressure side of the primary seal. The injected flush provides a clean, cool fluid at the sealing faces to prevent vaporization of the seal (process) fluid at the seal faces. Most pumps are self flushing. Since the seal flush piping is furnished with the pump and there is no instrumentation provided, a separate P & ID detail is not required. The following Plans are commonly used:

Plan 11 Recirculation from pump discharge through a flow control orifice

This plan is the default.

Plan 13 Line from pump stuffing box back to pump suction. Used for vertical in-line process pumps or Sundynes handling light hydrocarbons (sp, gr. <0.6).

Plan 22 Recirculation from pump discharge through a flow orifice and cooler. (See Plan 23.) If the pumping temperature is greater than 400ºF (200ºC), consult a rotating equipment specialist to determine what type of cooler arrangement is required. Over 400ºF, a water cooled seal flush cooler is highly susceptible to fouling from the deposits left behind when the small amount of water immediately in contact with the exchanger wall high skin temperature vaporizes. therefore, an air cooler or a combination of air cooler followed by a water trim cooler can significantly improve seal Mean Time Between Repair (MTBR).

Plan 23 Recirculation from a pumping ring in the seal chamber through a cooler and back in the seal chamber

Use this plan when the pumping temperature is over 300ºF (149ºC).

Note that if the viscosity of the pumped fluid is over 30 cP the pumping ring may not provide enough pressure to circulate the fluid though the exchanger (ex. Raw Oil in an FCC unit). In this case use a Plan 22 with and air cooled exchanger.

Plan 31 Recirculation from pump discharge through an orifice and cyclone separator

Use this plan for Sundyne pumps with pumping temperature under 300ºF (149ºC) and specific gravity over 0.600.

Plan 32 Fluid is injected into seal chamber from an external source

Use this plan for dirty or erosive services with a flush fluid compatible with the process.

Plan 41 Recirculation from pump discharge through an orifice, cyclone separator and cooler

Use this plan for Sundyne pumps with pumping temperature over 300ºF (149ºC) or pumps with pumping temperature over 650ºF (343ºC).

Plan 62 External water quench on atmospheric site of seal faces.

Use this plan for caustic service to prevent solids build up.

Include a separate detail on the P&ID only when the seal flush source is external (Plan 32).

6.2 Auxiliary Seal Flush Piping Plans

Auxiliary Seal Flush Piping Plans and piping details are also identified by a number found in API 610. Generally, an additional detail is required on the P&ID when an API Auxiliary Seal Flush Plan is specified. These plans show standard piping requirements for injecting seal flush fluid or nitrogen to the low-pressure side of the primary seal. Specify API Plan 52 for unpressurized seals with a seal reservoir, Plan 53 for pressurized seals with a seal reservoir, and Plan 54 for an external seal flush without individual seal fluid reservoirs for pressurized or unpressurized dual mechanical seals. Also use Plan 54 for nitrogen injection for non-contracting dry seals. See the following for a discussion of each of the API Plans and which detail to show:

a. Unpressurized Dual Seals

(1) General

Unpressurized dual seals have a buffer fluid or gas circulated between two sets of seal faces at a lower pressure than the process so the process fluid leaks into the chamber between the seals. This scheme is used to minimize leakage of process fluid to the atmosphere and when the process cannot be contaminated by an external fluid.

(2) Unpressurized Dual Seals API Plan 52 Guidelines

Reference Procedure for details. A P&ID for a particular process unit and the P&ID Procedures Manual shows P&ID unpressurized seal flushing system details such as "TL" or "TS" .

(3) Unpressurized Dual Seals API Plan 54

Use Plan 54 when an external source such as cooling water is circulated between the seals. No reservoir is required. Use this scheme primarily for caustic services.

(4) Unpressurized Dual Seals with dry containment seals API 72 and 75 or 76

Upon customer request a dry outer containment seal may be specified. Use Plan 72 along with Plan 75 if the process fluid is not 100% flashing at atmospheric temperature and pressure or Plan 76 if the process fluid is 100% flashing at atmospheric temperature and pressure.

b. Pressurized Dual Seals

(1) General

In the pressurized dual seal type arrangement, a pressurized fluid circulates through the space between two seals. The pressure of the sealing fluid is maintained 25 to 60 psi greater than the pressure at the pump side of the seal. Any leakage will be pressurized fluid through the inboard seal into the pump casing. The outboard seal provides seal of the pressurized fluid from the environment. Use API Plan 53 or modified API Plan 53 (provides for individual pressurized seal fluid reservoirs for each seal with the pressurization maintained by a gaseous source) to indicate this arrangement on Project Specification 501. Alternatively, use Plan 54 for an externally provided flush injected directly into each seal cavity without seal fluid reservoirs. Only use pressurized dual seals when contamination of the process stream by the sealing fluid is not of concern.

(2) Pressurized Dual Seals API Plan 53

In this arrangement, each pair of seals is provided with a pressurized reservoir. The pressure is maintained 20 to 60 psi higher than that the pump suction pressure by an external gas source, usually nitrogen. This eliminates the need for an external circulating seal flush system. The reservoir is filled with a seal flush fluid as specified in the pump specification. This fluid is cooled and circulated through the cavity between the mechanical seals by either a pumping ring or the action of the mechanical seals. Leakage of the seals either into the process or to the atmosphere may be determined by the loss of level in the seal reservoir.

Use P&ID detail "F" when pressurized mechanical seals are specified with API Plan 53, which was developed for power recovery turbines but may be modified for other services where API Plan 53 is required. Pressurization is maintained on the seal oil reservoir using nitrogen. Low-pressure and low-level alarms alert the operator to problems in the sealing system.

Also use P & ID detail F for unspared Charge Pumps when the pumping temperature is under 400ºF. If the pumping temperature is 400ºF or over (resid units), use detail O. This is similar to Plan F but also shows a Plan 32 inner seal flush to keep the inner seal cool and free of particles.

(3) Pressurized Dual Seals API Plan 54

This arrangement is similar to API Plan 53 but must have an external pressurized source for the seal. Seal reservoirs are not provided.

Use P&ID detail "D" when pressurized mechanical seals with external seal flush are specified on the pump specification sheet. A high-pressure sealing fluid from an external source is injected into the cavity between the two mechanical seals. If leakage occurs, it will be sealing fluid leaking into the process or out to atmosphere. Flow indicators and high and low flow alarms are shown into and out of the seal cavity to determine inboard seal leakage.

Use P&ID detail "A" when pumping fluids containing HF or detergent alkylate acid. A high-pressure sealing fluid is injected into the cavity between the two mechanical seals. In addition a fluid is injected as seal flush to the inboard mechanical seal. This flush leaks into the pumped fluid. The seal flush acts as a barrier to keep acid away from the seal parts, increasing the life of the seal. The type of fluid used is determined by the type of contamination that may be allowed in the pumped fluid. Flow indicators and alarms are shown into and out of the seal cavity to determine inboard seal leakage. Also an auxiliary seal arrangement is shown on the atmosphere end of the dual seals.

Non-contacting pressurized dry gas seals API Plan 74

Non-contacting gas lubricated seals have two sets of rotating and stationary faces similar to liquid lubricated pressurized dual seals. A gas (typically nitrogen from the refinery nitrogen header) is injected between the two sets of seal faces.

Revision Indication

Non-contacting gas lubricated seals may be specified up to an operating temperature of 500 F and up to a maximum suction pressure of 300 psig. (Note that the normal suction pressure must be 30 psi less than the normal nitrogen header pressure and the maximum suction pressure must be 5 psi below the normal nitrogen header pressure). If both of these conditions can not be satisfied and non-contacting gas lubricated seals must be used a mechanical amplifier must be provided to boost the Nitrogen supply gas pressure. Be aware the continuous nitrogen consumption required to operate the mechanical amplifier, which is simply a pneumatic piston, can be substantial especially if multiple pumping services are impacted.)

P&ID detail "DD" (dual dry) shall be referenced at the pump on the P&ID. This detail simply shows the nitrogen line with an isolation valve coming into the pump seal console, which is provided by the seal vendor.

7. Additional Sealing Requirements

All single seal arrangements require a non-sparking throttle bushing at the outer end of the seal to minimize and control leakage to the atmosphere if there is a severe seal leak. Dual seal arrangements require a non-sparking throttle bushing where seal chamber dimensions allow.

If there is over 50% aromatic hydrocarbon in the pumped fluid add the following note in the 501 Project Spec: Mechanical seal parts and packing shall be suitable for handling aromatic hydrocarbons. This note is required because some seal o-ring materials such as Viton are not compatible with aromatics.

All seal and auxiliary seal flush piping shall be stainless steel tubing with tube type fittings provided there is no chemical compatibility issue. If both HCl and H2O are present in the process fluid, use monel tubing for the seal flush. If trace HF acid is present use carbon steel.

If the pumped fluid is 100% MTBE, specify Kalrez 4079 o-rings.

All single and dual seal arrangements shall be supplied as a one-piece cartridge from the seal vendor.

All mechanical seals shall be designed to API Standards 682, Shaft Sealing Systems for Centrifugal and Rotary Pumps.

8. Special Requirements for Pumps with Sub-Atmospheric Suctions

If air leakage is not hazardous to the process, use single mechanical seals with API seal flush plan 11.

If air leakage is hazardous to the process and needs to be minimized:

For pumps where the normal suction pressure is over (-6) psig (9 psia), use single mechanical seals with API seal flush plan 11. The vacuum will not be low enough to pull air into the process.

Revision Indication

For pumps where the normal suction pressure is under (-6) psig (9 psia), do one of the following listed in order of increasing capital cost:

1. Use Single Mechanical Seals with “Crossover” API Plan 11 Seal Flush

The API Plan 11 seal flush will ensure that the operating pump’s seal is under positive pressure. The spare, idle pump’s seal however will be under vacuum and draw in atmospheric air. To prevent air from entering the idle pump, the API Plan 11 from the discharge of the operating pump should supply seal flush to both the operating and spare pumps. This will ensure that both seals have positive pressure.

Piping must be erected to connect each pump’s Plan 11 seal flush to the other pump’s inner seal flush connection. A single line connecting the A and B pumps with a locked open isolation valve should be shown on the P & I. Note that either pump could be the operating pump. The two pumps should be in close proximity to each other.

2. Use Pressurized Dual Non-Contacting Gas Seals

The nitrogen header should be connected to the barrier connection between the seals. The barrier outlet should be capped. The normal nitrogen header pressure should be minimum 30 psig above the normal suction pressure. It must be acceptable to leak a small amount (under 2 scfm) of nitrogen into the process. The barriers between the dual seals must be kept pressurized for the spare, idle pump to prevent air leakage. Use P & ID detail “DD”. See Procedure

3. Use Pressurized Dual Mechanical Seals

API seal flush plan 53 or plan 54 if the pressurized barrier fluid is supplied from a header. Use P & ID detail “F” or “D”.

ATTACHMENT 1

Examples of Sealing Requirements for Centrifugal Pumps

SERVICE	SEAL TYPE	API AUX. FLUSH PLAN	P&ID DETAIL	SEAL BUFFER FLUID/ALARM TYPE
Light hydrocarbon	Unpressurized	52	TL	Per Spec /PAH,LAH
0.1 wt% benzene	Unpressurized	52	TA	Per Spec /Note A
25 wt% C6 to C9 Aromatics	Unpressurized	52	TA	Per Spec/ Note A
100 ppm H2S	Unpressurized	52	TH	Per Spec/Note A
Above auto-ignition temp.	Unpressurized	52	TA	Per Spec/Note A
Aromatics and Sulfolane	Unpressurized	52	TS	Per Spec /LAH
Lean Sulfolane Solvent (ED Only)	Pressurized	53	F	Sulfolane /LAL. PAL
Water & Ovhd	Pressurized	None	D	Water /FAL
HF Acid	Pressurized	None	A	Per Spec/FAL,FAH
Detergent Alkylate Acid	Pressurized	None	A	Per Spec/FAL,FAH
Detergent Alkylate Benzene Rich	Pressurized	None	D	Paraffin /FAL,FAH
Caustic	Single	62	TC
Caustic Sunflo Type	Pressurized	54	DC	Water /FAL,FAH
Vacuum (No air into process)	Single or Pressurized or Non-contacting dry gas	None 53 or 54 54	C F or D DD	Per Spec/LAL or FAL,FAH
Hyd Power Recovery Turbine	Pressurized	53	F	Diesel /LAL,PAL
Methanol	Unpressurized	52	TA	Per Spec /LAH,LAL
Unspared Charge Pump	Pressurized	53	F or O	Per Spec/LAH
1Wt% highly toxic chemicals*	Unpressurized	52	TA	Per Spec /Note A

* See , Attachment 2

Note A: A high pressure or high level alarm may be required to warn of liquid leaking across the inner seal, and the selection depends on the degree of flashing. Vapor pressures are described in units of psia at 100ºF, and the breakpoint between high and low vapor pressures is considered to be 40 psia. If vapor pressure is low and there's no flashing, a level alarm will be required. For high vapor pressures with 90 percent flashing, a pressure alarm will be required. If the vapor pressure is high at the pumping temperature and low at 100ºF, both pressure and level alarms will be required. For a non-flashing fluid, a high level alarm will provided a faster indication of an inner seal leak than a high pressure alarm.

In addition to the seal reservoir alarms mentioned, all seal pots also have a low level alarm to warn of outer seal leakage and loss of reservoir fluid.

ATTACHMENT 1 (continued)

Examples of Sealing Requirements for Centrifugal Pumps

SERVICE	SPECIAL AUX SEAL	VENT	DRAIN	STD. DRAWING
Light hydrocarbon	None	Relief Header	None	8-136
1 wt% benzene	None	Relief Header	Closed Aromatics	8-137
25 wt% C6 to C9 Aromatics	None	Relief Header	Closed Aromatics	8-137
100 ppm H2S	None	Relief Header	Safe Location	8-135
Above auto-ignition temp	None	Relief Header	Safe Location	8-137
Aromatics and Sulfolane	None	Vent Header	Sulfolane Sump	8-137
Lean Sulfolane Solvent	None	None	None	8-137
Water & Ovhd	None	None	None	None
HF Acid	Provide	Acid Relief Header	None	None
Detergent Alkylate Acid	Provide	Acid Relief Header	None	None
Detergent Alkylate Benzene Rich	None	None	None	None
Caustic	None	None	None	None *
Caustic Sunflo Type Pump	None	None	None	None **
Vacuum (No air into process)	None	None	None	None
Hyd Power Recovery Turbine	None	None	None	None
Methanol	None	Relief Header	Closed Aromatics	8-137
Unspared Charge Pump	None	None	None	None
1Wt% highly toxic chemicals***	None	Relief Header	Closed Aromatics	8-137

* Reference the following note in the 501 Project Spec. (Note 5):

Revision Indication

Provide clean water flush on atmospheric side of seal faces to prevent solids build up.

** Reference the following note in the 501 Project Spec.:

Pump vendor shall provide buffer in and buffer out (BI/BO) tap connections on the pump stuffing box. Provide a clean water flush between seals to avoid caustic buildup and leakage to atmosphere. A rotameter/transmitter (by others) for control center indication and alarm will be located in the inlet piping from each seal.

*** See ”, Attachment 2

. ATTACHMENT 2

Links to P & I Details and Standard Drawings

Details TC, TA, TL	f300001
Details F and O
Details TS, TP, D
Detail A
Detail TH
Standard Drawing 8-136	Pump Vent System for High Vapor Pressure Liquids
Standard Drawing 8-137	Pump Drain System