Designing pump seals

[NPC5]

When designing pump seals, how does the max suction pressure come into play? In the case that the pump is able to see max suction pressure I suppose you would be concerned that a seal would fail and the process fluid gets exposed to the atmosphere? What sort of scenarios should one be aware of when designing a seal flush system? Thanks in advance!

 

[GPRnD]

Maximum suction pressure matters because (depending on the pump type), it determines the upper limit of the pressure the mechanical seal will see.

The exception to this is with some pump designs and seal plans, such as plan 13, where the mechanical seal will see both the suction pressure and a portion of the pump differential head.

In turn the maximum pressure that the seal needs to be designed for will greatly affect the seal design and cost. If you ever have the chance to look at a seal designed for a boiler circulation pump (with suction pressures of 2500+ psi), you'll understand what I mean.


Picking the right seal flush plan depends on so many factors it is impossible to list them all here. I'd recommend reading up on the subject and referring to a standard like API682 for help. However the main factors to consider are:

Toxicity, flammability of the fluid
Temperature of the fluid
Fluid vapor pressure margin
Heat generation of the seal
Potential for polymerization/solids build up
Solids content, abrasives
Vapor/air removal

If you can comment on your specific fluid parameters and application I can offer some suggestions.

 

[NPC5]

Bradshsi,

Thanks so much for your help.

I am actually looking at the design of the barrier fluid being used. The seal flush plan, flush fluid, and barrier fluid have all been designated. I am in the process of working with the pump vendor to determine he barrier fluid supply pressure needed at the seal. My supervisor brought up the topic of maximum suction pressure being a factor and how the seals needed to be designed for this situation.

For a centrifugal pump are the seals designed to withstand the maximum suction pressure of the pump? I have tried to think of other ways that max suction pressure affect the seals but this is all I have come up with. My supervisor made it seem as though there was more to it though. Is there more to it than this?

 

[GPRnD]

Seals need to be designed to withstand at least the maximum suction pressure of the pump.

Some standards such as API 682 specify limits (in the case of that standard, anything from 315 to 615 psi depending on which category they are designated).

The pump suction pressure affects the seal heat generation and life. I don't know if that was what your supervisor was alluding to ?

 

[CJKruger]

If you have a external flush, then the flush supply pressure must be 25 psi above the seal champer (stuffing box) pressure. The seal champer pressure depends on the suction pressure.

Some companies require that the external flush must be able to enter even when the pump is at maximum suction pressure. Sometimes this is difficult to achieve though.

 

[NPC5]

Bradshsi, in what ways does the pump suction pressure affect the seal heat generation and life? I don't have much experience with pumps and would like to learn as much as possible.

CJKruger, do companies want the external flush to be able to enter at maximum suction pressure for any specific reasons? Is this to help protect the seals or something?

Thanks to both of you for your help!

 

[GPRnD]

The higher the pressure, the more heat is generated and the higher the flush rate is needed to carry that heat away.

The seal company can give you a tabulation of pressure vs heat generation for the model of seal you are using.

It affects seal life because the higher pressure results in more distortion of the seal components and a thinner fluid film between the faces. The combination of these two means that physical contact of the seal faces will occur more often resulting in more wear.

The external flush has the purpose of carrying away vapor and excess heat from the seal faces. I has to be higher than the suction pressure or you would not get any flush (no pressure differential to drive the flow).

25 psi is a good baseline rule of thumb. However if the process fluid has a v high vapor pressure, you may need significantly more.

 

[longeron]

Maximum suction pressure is really only part of what is needed. The pump OEM should be able to provide information regarding the discharge pressure for the range of operating conditions as well as the seal chamber pressure. The seal chamber pressure depends upon the differential pressure across the pump as well as certain pump design factors.

You mentioned a flush fluid and a barrier fluid. It sounds to me as if the seal vendor has suggested using a fluid other than process to inject into the seal chamber as well as using a dual pressurized seal.

API Plan 32- Obviously the injected flush fluid must be compatible with the process, must not be injected at a rate that would dilute the process fluid, and be at a higher pressure than the seal chamber. This flush plan us often used when the process fluid has solids that would damage the seal faces, when the process fluid has poor lubricating properties/has a high vapor pressure, or when the process fluid is otherwise nasty- corrosive, toxic etc. Bradshsi makes a good point about the vapor pressure. Vaporizing fluid in a seal chamber is never good. The external flush fluid can help increase the seal chamber pressure as well as cool the seal reducing vaporization. A throat bushing in the seal chamber can be sized to help regulate flush fluid flow as well as seal chamber pressure. A box pressure of 25 psi above vapor pressure is a rule of thumb. Depending upon the process fluid and conditions it may not be enough.

API Plan 53 or 54- The pressure between the two mechanical seals must be higher than the seal chamber pressure, else process fluid will leak into the barrier fluid during operation of the pump. These flush plans rely upon some method of supplying a high pressure fluid between two mechanical seals in the pump. This flush plan is often used when the process fluid is nasty (as mentioned above, or is otherwise hazardous to personnel. Keeping the barrier fluid pressurized means that the process fluid cannot escape to the atmosphere. The 25 psi above seal chamber pressure is another rule of thumb with barrier fluid. 25 psi is usually a good enough margin to deal with changes in process conditions. This should also be reviewed- especially if you expect wild swings in process pressure. If so- discuss the use of an API Plan 53C flush plan with your seal vendor. A 53C uses a "piston pot" that references discharge pressure to automatically adjusts to changes. Or go for Plan 54- a lube oil skid used to support the seal.

Heat and pressure are always considerations with mechanical seals. Heat is usually dealt with by cooling the seal faces. In this case the Plan 32 injected flush fluid is usually chosen so that it is cooler than the process so it removes heat from the seal as it flows through the seal chamber. The barrier fluid (Plan 53 or 54) is chosen to be a good lubricator with good heat transfer properties. More often than not the barrier fluid flows through a circuit that includes a heat exchanger to remove heat generated by the seal faces. Pressure does distort seal faces, but there are ways to strengthen seal faces. Consult your vendor.