Seal-Chamber-Pressure.pdf

Seal Chamber Pressure

John Crane Mechanical Maintenance Training Center

By Gordon Buck John Crane Inc. Baton Rouge, LA for the John Crane Mechanical Maintenance Training Center

Estimating the Pressure in Pump Seal Chambers The pressure in the pump seal chamber must be known before the proper seal and flush system can be selected. For some pumps, the seal chamber pressure is the same as the pump suction pressure; for others, it may be near discharge pressure. The exact value depends on the fluid, flowrate, design and condition of the pump. Often, accurate information is not available even from the pump manufacturer so the best way to determine the stuffing box pressure is to measure it. This procedure shows how to estimate the seal chamber pressure when measured data is not available and the pump manufacturer cannot or will not help. These symbols are used throughout this paper: Pb = Pressure in the seal chamber Ps = Pump suction pressure Pd = Pump discharge pressure

Single Stage, Single Suction, Overhung Process Pumps Overhung process pumps are by far the most common type of pump used in the process industries. These pumps can be either horizontal or vertical (inline) and are usually single stage, single suction. The seal chamber is located near the back of the impeller. Balance Holes/Back Wear Rings The majority of overhung process pumps use balance holes and back wear rings in the impeller to decrease the thrust load on the bearings. These features also set the seal chamber pressure. The exact pressure is a function of the wear ring clearance as well as size and location of the balance holes. It can be estimated as: Equation 1: Pb = Ps + .15 (Pd - Ps ) Important: The same pump model may be available with or without balance holes. Example: A single stage end suction pump that uses balance holes and back wear rings has a suction pressure of 10 psig and a discharge pressure of 200 psig. What is the seal chamber pressure? Pb = 10 + .15 (200 - 10) = 38.5 psig Without Balance Holes Balance holes may not be needed to reduce the thrust load at higher suction pressures. Also, sometimes balance holes are omitted in an effort to increase pumping efficiency. For these pump designs, the seal chamber pressure is very close to discharge pressure: Equation 2: Pb = Pd Important: The same pump model may be available with or without balance holes. Seals cannot be flushed using injection from discharge. Example: A single stage end suction pump does not use balance holes. Suction pressure is 10 psig and discharge pressure is 200 psig. What is the estimated seal chamber pressure? Pb = 200 psig

Seal Chamber Pressure

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Pump-Out Vanes Some pumps use pump-out vanes instead of balance holes to reduce thrust. This is especially popular on ANSI pumps. Impeller clearance from back plate is critical to the efficiency of these pump-out vanes. The seal chamber is slightly above suction pressure: Equation 3: Pb = Ps + C (Pd - Ps ) For most of these pump designs, use C = 0.1 but see notes 1 and 2 below. Note: 1.Goulds recommends C = .3 if the impeller is at minimum diameter but C = .1 if the impeller is at maximum diameter. 2.Durco provides seal chamber pressure information on the pump performance curve. The performance curve usually indicates pressures lower than Equation 3. Seal chamber pressure is near suction pressure at no flow, maximizes near BEP, then decreases. Example: A single stage, end suction pump that uses back pump-out vanes has a suction pressure of 10 psig and a discharge pressure of 200 psig. What is the estimated seal chamber pressure? Pb = 10 + .1 (200 - 10) = 29 psig

Single Stage Double Suction Pumps Double Suction impellers can be provided for many different pump constructions including single stage overhung, single stage between bearings, horizontal multi-stage and vertical multi-stage. Next to the single stage, single suction, overhung construction, the next most common pump construction is probably the single stage, double suction impeller placed between bearings. This design requires a mechanical seal on each side of the impeller. The seal chambers are placed adjacent to the suction eye; therefore seal chamber pressure is the same as suction pressure. Equation 4: Pb = Ps Important: Seals in these pumps cannot be flushed using API Plan 13. Example: A single stage, double suction pump has a suction pressure of 10 psig and discharge pressure is 200 psig. What is the estimated seal chamber pressure? Pb = 10 psig

Multi-Stage Pumps Multi-stage pumps can be mounted horizontally or vertically. They might use single or double suction impellers. Balance holes, wear rings, balance drums, impeller orientation and many other features are used to control thrust and seal chamber pressure. For many of these pumps, the most important effects on seal chamber pressure are produced by the location of the seal chamber with respect to the impeller and the use of a balance line. Two-Stage Horizontal Pumps There are two distinct configurations of two stage horizontal pumps. Each has two seal chambers. The more common design has the impellers “back to back”. In this design, the seal chambers are located near the impeller suction eyes. This means that one seal chamber is at pump suction pressure. Equation 5: Pb = Ps The other seal chamber is at second stage suction (first stage discharge). Equation 6:

2

Pb = Ps + .5 (Pd - Ps )

Seal Chamber Pressure

United and Byron-Jackson each make a two stage pump with impellers “eye to eye”. In this design, the seal chambers are located near the back of the impeller This means that one seal chamber is at the discharge pressure of the first stage. Equation 7:

Pb = Ps + .5 (Pd - Ps ) The other seal chamber is at pump discharge pressure. Equation 8:

Pb = Pd

Important: Various bleed off lines are frequently used to lower the seal chamber pressure. See the section on Multi-Stage pumps. Example: A two stage United Model THF (impellers placed “eye to eye” has a suction pressure of 10 psig and a discharge pressure of 400 psig. What is the estimated seal chamber pressure? The low pressure chamber is at (using Equation 7): Pb = 10 + .5 (400 - 10) = 205 psig The high pressure chamber is at (using Equation 8): Pb = 400 psig Multi-Stage Horizontal Pumps Multi-stage pumps are used to develop high pressures, but the seal chamber is not necessarily at a high pressure. Most horizontal multi-stage pumps (boiler feedwater, pipeline) have two seal chambers: a “low pressure” chamber at suction pressure and a “high pressure” chamber at a pressure between suction and discharge. The low pressure chamber is at: Equation 9:

Pb = Ps

A balance line is used to reduce the pressure in the high pressure seal chamber. When the pump is in good repair, the high pressure chamber is around: Equation 10:

Pb = Ps + 75

Important: If the balance line is not used or if the close clearance throat bushing is not maintained, then the pressure in the high pressure seal chamber tends to approach midway between suction and discharge pressure. That is: Equation 11:

Pb = Ps + .5 (Pd - Ps ) Example: A multi-stage pipeline pump has a suction pressure of 350 psig and a discharge pressure of 1200 psig. What is the estimated seal chamber pressure? What would the pressure be if the balance line were not used? For the low pressure chamber, use equation 9: Pb = 350 psig For the high pressure chamber, use equation 10: Pb = 350 psig + 75 = 425 psig Without the effects of the balance line, the high pressure chamber can reach: Pb = 350 + .5 (1200 - 350) = 775 psig

Seal Chamber Pressure

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Multi-Stage Vertical Can (Turbine) Pumps In these pumps even though the individual impellers may have balance holes and back wear rings, the seal chamber is located in the discharge nozzle. Therefore the seal chamber pressure would be at pump discharge pressure except for the effects of a balance, or bleed-off, line. When the pump is in good repair, the seal chamber is around: Equation 12: Pb = Ps + 75 Important: If the balance line is not used or if the close clearance throat bushing is not maintained, then the seal chamber is at discharge pressure. In this case, flush plans using injection from pump discharge (API Plan 11, 21, etc..) cannot be used. Example: A multi-stage vertical can pump has a suction pressure of 350 psig and a discharge pressure of 1200 psig. What is the estimated seal chamber pressure? What would the pressure be if the balance line were not used? With the balance line: Pb = 350 + 75 = 425 psig Without the effects of the balance line, the chamber can reach: Pb = 1200 psig

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Seal Chamber Pressure

Table 1 - Classification of Various Pumps This table summarizes the classification of the standard construction of the more popular pump models made by various manufacturers. Custom designs and variations are common. Pump Manufacturer and Model

Equation

Remark

Bingham CAP, CAD, CVA, HVA CD/CDA, HS, HSA/HSB/HSD MSD, CP/CPA BA, BK

1 (or 2) 4 9, 10 12

usually will have balance holes

1 (or 2) 4

usually will have balance holes

Byron Jackson SJ, SC7, IN DSJ/DSJH CMX, DVMX Durco Mark I, II, III

3

use C =.1 (see note 2 on page 2)

Goulds 3196, 3996 3700, 3910 3404/5, 3410, 3415, 3420, 3620 3640 3300, 3600, 7200, 7500 VIT, VIC

3 1 (or 2) 4 5, 6 9, 10 12

see note 1 on page 2 usually will have balance holes

Ingersoll-Rand A Line S, J Line, SL VOC, HOC DA

1 (or 2) 4 3 9, 10

usually will have balance holes

Pacific SVCN, SPCN HVC ROV UXN, BFI, CB

1 (or 2) 4 5, 6 9, 10

usually will have balance holes

United TC, VP, VIP DVS, DSTC, DVSHF THF VCD MSN, WMSN

1 (or 2) 4 7, 8 12 9, 10

usually will have balance holes

Union H, SRV, VLK, VCM, ULK, Uniline 1 (or 2) HOL 4 M 9, 10 VCP 12 Wilson Snyder ESN, ESW, VIL, VILS DV IVT Seal Chamber Pressure

1 (or 2) 4 12

use C = .1

usually will have balance holes

usually will have balance holes

5

Notes:

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Seal Chamber Pressure

John Crane Inc. Mechanical Maintenance Training Center 529 West Golf Road Arlington Heights, Illinois 60005 Tel: 847-364-0577 Fax: 847-364-1677

© John Crane Inc.

Seal Chamber Pressure MMTC 901-3 March 2000