Stuffing Box Pressures for Non-API Multistage Centrifugal Pumps 1

[mobgma]

Afternoon

Looking for any help on calculating stuffing box pressures on a multistage centrifugal pump with an equalizing line.

I am told (by pump/seal mfg's)that you cannot calculate a true value as there are simply too many variables to take into consideration and as such the industry assumes that the stuffing box pressure = the suction pressure through the use of an equalizing line. Please see the attached technical reference on how to calculate stuffing box pressures for different pumps.

I am looking for some clarification if the above is true. Is there no real way to calculate a true value?

I have heard of some theoretical ways to calculate these values depending on the number of impellers, pump design, pump tolerances, whether equalizing lines are used, location of stuffing box in relation to the area where the impellers is turning, type of axial thrust balancing employed, peripheral speeds + pressure in seal areas combined into a PV value to determine frictional heat developed, liquid temperatures, etc etc.

Any help or advice is muchly appreciated.

 

[bk19702]

The formulas you have in the document are fairly consistent approximations in that the lower pressure seal chamber should be suction pressure; the higher seal chamber pressure being roughly equivalent to suction pressure + 75 psig. What makes these values difficult to pinpoint on a multi-stage pump is that the internal conditions of the equalizing line, wear rings, and thrust balance device are often not known. As these elements wear and clearances increase, you will most likely experience an increase in seal chamber pressure on the high pressure side of the pump. Typically the pump OEM can provide some insight as to what these values should be. If it's a new pump, the seal chamber pressure can be recorded during the performance test. On exisiting installations, I have mounted gauges to old lantern ring connections on the casing to get 'real time' seal chamber pressure values, then comparing these to what calculated numbers should be. This is not a bad practice if the casing can accommodate it, as monitoring these pressures will provide some indication as to the condition of the pump internals.

 

[Pumpsonly]

The formula given in the attachment are just general guide line for the seal chamber pressure as it is without considering the required seal flushing Plan. example, the seal chamber pressure at the suction side will not be suction pressure if Plan 11 is used.

The pressure at the higher pressure side will very much depending on the construction of the pump, and the balancing line dp use by the pump manufacturer. The balancing line dp are based on the pump head and fluid properties at the design condition. Operating away from this conditions will change all the parameters.

For a more critical application such as low SG and high vapor pressure liquid, it is best to consult the pump vendor. Too low a pressure in the seal chamber may cause the liquid to flash and too high pressure may cause insufficient flushing liquid flow over the seal faces.

 

[NGLENGR]

The stuffing box pressure is highly dependant on the flush plan and the clearance of the box bushings as well. We have some pumps that use Vespel bushings with a high stage flush stream to maintain vapor pressure margin for the seals.

You can calculate an order of magnitude estimate by having the curve and flush stream data handy with reasonable assumptions of head loss across the system. It isn't rocket science, just time consuming. I've had seal vendors calculate theoretical stuffing box pressures before and they are usually within +/-10% of the test pressure (using a gauge on the box itself)

Highly recommend that you consult both the pump and seal vendor and get them to communicate so that you have adequate stuffing box conditions.

 

[mobgma]

Thanks very much for your input. I appreciate all your responses.