Mechanical Seal API Plan53B

[S_Thy]

We are facing a problem with low MTBF of mechanical seal i.e. less than 3 years. Seal installed for Effluent Water Pump is Eagle Burgmann make (DF-H75F-D/95-E2) with a seal of API Plan 53B. If we see all the operational parameter of seal system are well with in range (Barrier Fluid Pressure, Mech. Seal/Barrier Fluid temperature, Barrier Fluid Oil Quality, Pump vibration & suction and discharge pressure also stable) as per design and as recommended by OEM. But we are facing random seal failure in pumps after a period of 1.5 years after new seal installation. Only operational set point we observed deviating from the recommended value is the accumulator pressure is higher by +6 barg (10 bar) from recommended pressure of required 4 barg.
Is this may be the possible reason for seal failure or we have to look in to some other area also?
Thanks

 

[JJPellin]

We have not had good results with our Plan 53B systems. We have about 20 of these in operation. A lot of our failures have been associated with these problems:

1. Barrier fluid contamination – Water contamination may have been introduced by the operators when they added fluid. Process contamination may have gotten into the system during periods of low barrier fluid pressure. We may have failed to do a good enough job of cleaning out the system after a seal failure.

2. High or low barrier fluid pressure – Our systems seem to have a lot of pressure variability based on sun or shade, day or night, winter or summer. We have had to add insulation and sun shades in some cases.

3. Freeze up – We have seen these problems combine when water would get into the system, accumulate at a low point and freeze, blocking the proper function of the accumulator, resulting in extremely high pressure when the off-line pump was started up.

Because of the poor reliability we have seen with Plan 53B, we likely won’t be installing any more of these for a long, long time.


Johnny Pellin

 

[S_Thy]

Thanks JJPellin,

Thanks for your inputs. based on your inputs, we will compare the API seal plans resulting in higher seal failure issue. From the seal flushing system side we have considered the auto refilling unit start-up well above (+1 barg) the required stuffing box/seal chamber pressure.

Considering seal failure identification, i think with API Plan 53B it is easy to locate, which mech. seal has failed i.e. (DE or NDE), but as in case of common seal supply system as in API plan 54, it is difficult to detect the which side of seal has failed and some time both DE & NDE side mech. seal of pump has been replaced in confusion. This seems me to be advantage of having a API Plan 53B. Kindly correct me on this, if i am wrong.

Thanks

 

[JJPellin]

Your pressures do not seem so high as to require a Plan 53B. I would consider a Plan 53A before I would jump to a Plan 54. You are correct, with a Plan 54, it is not immediately obvious which seal has an internal leak. But, a simple test can be done to determine which to replace. Block in each seal on the inlet and outlet for a few seconds. The seal which is leaking will bleed down the trapped pressure between the block valves quickly. A good seal with no internal leakage, the pressure will build as the trapped fluid heats up and expands. For final effluent water, I am not sure why you need a pressurized dual seal at all. A Plan 52 could provide a simpler system and better reliability.

Johnny Pellin

 

[S_Thy]

Hi,
I need concern over following points related to Mechanical Seal:
1- As per Mechanical Seal Flushing Plan API 54, barrier fluid pressure should be higher by max +2 barg from pump stuffing box pressure. My doubt is that, what if barrier fluid pressure maintained by +6 barg higher than stuffing box pressure?, is t going to reduce the seal life?.
2- Is there any standard for assigning the Barrier fluid pressure Low-Low set point i.e. How much higher than pump stuffing box pressure, so that process fluid should not enter seal circuit.
Thanks

 

[JJPellin]

These pressures do not seem high enough to cause any major reduction in mechanical seal life. You did not describe the seal in detail. But, even if it is bellows seal, it is likely rated for continuous operation up to about 20 barg. We operate our Plan 54 systems at about 2 barg above seal chamber pressure. In theory, you should not see process contamination of the barrier fluid unless that differential goes negative. Of course, that assumes that you actually measure seal chamber pressure and know exactly what that pressure is. If you are assuming the seal chamber pressure based on pump manufacturer's literature, you need a wider margin because it may not be exactly the pressure they told you it would be. I would try to never drop the barrier fluid differential pressure below 1 barg above seal chamber pressure.

Johnny Pellin