Barrier Fluid Pressure Calculation
Barrier Fluid Pressure Calculation
(OP)
I am looking to effectively and accurately determine the settings on double mechanical seals with water flushes as the barrier fluid for some applications in my plant and I need some help in determining the correct back pressure on the seal. I have seen vaious formulas for estimating stuffing box pressure and I know that dead-heading a pump is the most accurate way to determine the pressure that I must overcome with the barrier fluid. Additionally, Durco (the brand of the majority of my pumps) includes a small curve with every performance curve that shows "Stuff Box Head Above T.S.H.". My question then becomes, what does the "TSH" stand for in this curve? I can't figure out if it is Total Static Head, Total System Head, Total Shutoff Head, or something else! Thanks for any advice you may have.





RE: Barrier Fluid Pressure Calculation
http://virtualpipeline.spaces.msn.com
RE: Barrier Fluid Pressure Calculation
RE: Barrier Fluid Pressure Calculation
You should clarify with Flowserve (Durco) for the meaning of Stuff Box Head Above T.S.H." and clarify the seal chamber pressure.
For Barrier Fluid Pressure Calculation and what fluid you uses as Barrier Fluid, you need to contact Mechanical Seal Specialist to determine what kind of Double seal you have (It is real Double seal, Tandem seal, etc) what mechanical seal plan you use (available are Plan 52, 53A, 53B, 53C and 54). And pressure calculation is different for each arragement.
A little explanation for example if you have a Double Suction Pump with suction pressure of 30 psi with plan 52.
Your buffer fluid needs to be at atmosphere pressure.
In the same situation with Plan 53A, you should need more the atmospheric levels.
For more reference you could get API 682 3edition that covers this topics.
RE: Barrier Fluid Pressure Calculation
Ash Fenn
www.cdrpumps.co.uk
RE: Barrier Fluid Pressure Calculation
In the past this rule (about 80%) was very close for Centrifugal Overhung Pumps.
Pump Manufacturers realized that thrust bearing life and seal life was afected with this high pressure. Since this pressure tends to push impeller to suction side.
New designs incorporates impeller balance holes, balance lines, reverse impeller vanes, throat seal chamber bushing to reduce this pressure and axial load in pump.
Also, rule (about 80%) for double suction pumps and vertical pumps is not applicable.
Todays Industry uses double mechanicals seals in toxic and explosive (hydrocarbon volatile) fluids, so it is very advisable to know what exactly happen and ask to specialists.
RE: Barrier Fluid Pressure Calculation
If I recall the old rules of thumb correctly, for a typical ANSI style end suction pump, it was suction + 10% of differential or suction +25% differential, depending on whether the impeller had balance holes or back pumpout vanes, but I can't remember which was which.
Helpful as ever, I remain.
RE: Barrier Fluid Pressure Calculation
The bigger issue is then at what pressure should the water injection be. This does rely upon what the box pressure would be during normal operaton and the reason you've installed a Plan 32 to begin with.
As has already been discussed box pressure can be estimated, there are several equations out there depending upon the kind of pump, impeller, and seal chamber. Your OEM should be able to help you there (especially if you've got a Sundyne). Your seal vendor should also be able to help estimate. Worst case- you over-estimate the barrier fluid pressure and turn off the water injection to the pump while monitoring a pressure gauge in your Plan 32 piping, lantern ring connection, or gland plate.
If the water was installed to cool the seal, you'll need to calculate the flow rate required and know your throat bushing dimensions. If the water is to be used to exclude solids you'll need to know the bushing dimensions, calculate an appropriate velocity through the bushing and translate that into flow.
If your bushing clearance is too loose talk to your seal vendor they will have some solutions for that problem too. IMHO you'd find a way to make the bushing part of the seal so it is replaced everytime you change the seal.
RE: Barrier Fluid Pressure Calculation
In this instance, you will set your barrier pressure above your Plan 32 injection pressure (water pressure). Use the same 30 psi rule of thumb in this case.
I also agree that the bushing should be integral to the seal design to maintain the integrity during changeouts. I try to incorporate this into the seals I design/apply when the application calls for it. Your optimum flow velocity for solids exclusion is roughly 15 ft/s, so target that number in your calculations when evaluating the bushing clearance. You can run a composite bushing material (like a Vespel) to obtain a tighter clearance if need be.