Barrier Fluid Pressure Calculation

[mathieu1263]

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.

 

[BigInch]

Total shutoff head. The pump mfgr wouldn't know any of the other values.

http://virtualpipeline.spaces.msn.com

 

[BenThayer]

your best bet is to work your seal supplier. any reputable supplier should be able to help you with this information.

 

[PumpSmart]

mathieu1263,
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.

 

[pumpking]

The rule of thumb for stuffing box pressure is 80% of discharge pressure + suction pressure, then set flush at 0.5-1 bar above this figure. It is always advisable to speak to seal manufacturer, as we have seen all too often that people destroy seals in a few hours because they over/under pressurise the barrier system

Ash Fenn

http://www.cdrpumps.co.uk

 

[PumpSmart]

mathieu1263, pumpking,
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.

 

[TenPenny]

In the cobwebs of my mind, something tells me that TSH refers to total suction head, because the pressure in the stuffing box is typically a percent of the differential plus the suction head.

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.

 

[longeron]

Reading your original post it sounds like you are utilizing both a dual pressurized seal and an external water flush for the stuffing box. If that is the case, this is a much simpler problem. The seal must be pressured above the water injection. Most seal vendors will suggest 25 to 30 psig or 2 bar above this for the Plan 53 or 54 system.

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.

 

[bk19702]

Rule of thumb for barrier fluid pressurization is 30 PSI above max seal chamber pressure. As someone already stated, it is preferrable to obtain calculated seal chamber pressure from the OEM, otherwise you can try to calculate yourself but you must be wary of the pump design. For a Durco pump (Mark I, II, etc.), you can use 10% of the differential pressure + suction to calculate the seal chamber pressure.

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.