Measure seal cxhamber pressure on test bed - why? 3

[JoeySoap]

I have taken over a job that is over half way through and the company has ordered a raft of API610 centrifugal pumps of various sizes for an upgrade project from one vendor. As an additional requirement to API610, the company has asked for the seal chamber pressure to be meaured and recorded on test (mostly tandem seals used). Nobody at the company or the pump supplier can give a solid reason why this should definately be carried out and why it has been asked for. For some of the pumps, the vendor states there is no practical way to do this on test. What do the experts think please? Should I waive the requirement where the vendor thinks it is too difficult to do? The pumps range in size from small up to about 750kW and various discharge pressures upto about 60 bar. Thank you.

 

[JJPellin]

I like to document the seal chamber pressure when possible. For centrifugal pump, you often see people use a "rule-of-thumb" to estimate seal chamber pressure. But, depending on the configuration of the wear rings and balance holes, there can be quite a lot of variation. If you plan to use API plan 52, it may not be critical. But, if there is any chance that any of these pumps might end up with Plan 32, 53 or a gas seal, then this pressure is key. I don't understand how any manufacturer of an API pump could claim that this is difficult. They have to provide the option to inject seal flush into the seal chamber. All you have to do is connect a pressure gauge to an unused seal flush connection and monitor the pressure during the test run. If the purchaser does not allow for unused process connections, then they have to make a decision. Either allow a seal flush port with a plug or blind flange or skip the seal chamber pressure test. If the pump does not have a connection suitable to monitor chamber pressure, it is likely that the seal gland does.

Johnny Pellin

 

[NGLENGR]

Joey,

I'm gonna add a +1 to JJ's statement. In my world, we pump light hydrocarbons (ethane, propane, butane) which have high vapor pressures. The risk of having product flashing at the seal faces is a serious one, since having a liquid seal running on a film of gas is not an optimal condition.

I have done some testing of seal chamber pressures by putting gauges on the stuffing box ports off the balance line (BB3 pump). It's not perfectly accurate, but it gives me a reasonable guess to say if I'm getting a low vapor pressure margin.

There are several "rules of thumb" for the seal chamber pressure, but they are based on some broad assumptions about the pump and the configuration of the flush, balance lines, and bushing clearances. I've seen some fairly large imbalances between seal chambers on multi-stage BB3 style pumps due to that.

 

[JoeySoap]

Thank you both, that's helpful. They're mostly plan 53B and plan 75 systems from Eagle Burgmann.

 

[NGLENGR]

Ah, I'm very familiar with the E-B plan 53B systems. They're excellent, but I have had some issues with barrier fluid loss via the secondary seal o-rings. Our system runs at ~1300 psig barrier pressure, so the high delta between the barrier fluid and the atmosphere is causing the o-ring to extrude over time.

This is combined with the changing dynamic loads since the pump is VFD driven and sees a variety (C2s all the way to C5+) of product densities.

We have yet to have a "seal failure" in the sense that product was allowed to pass the primary seal, and the few seals we have replaced were usually in a near-perfect condition.

 

[flexibox]

I agree with NGLENGR however if you are using Plan 53B on your pumps chamber pressure is not relevant to seal performance. The only relevance relates to determining the Pre-Charge pressure in the accumulator (Plan53B) Review the P&ID carefully ensuring the Check valve is upstream of the accumulator as I have seen a bad design resulting in overpressurization due to heat expansion and no relief. Also check that the seals can be properly vented. Check the tubing size and ensure it is 3/4" with sweeping bends not elbows.

 

[JoeySoap]

Thanks Flexibox, you're right, ofcourse.

 

[NGLENGR]

I'd say the biggest concern with the 53B is your margin above suction pressure. The concern is if you run a high product suction pressure that falls within 25 psig of your barrier pressure and can fluctuate quickly for whatever reason, you might run the risk of a pressure reversal and allow product leakage. A 53C is the typical way around this, but the piston accumulators have their own challenges.

Flexibox is correct on all his points. Proper pre-charge on your accumulators, good thermal expansion allowance (our barrier pressures can range about 150 psig during a daily thermal cycle), and a way to vent the system are all necessary.

If you can get a good set of simple instructions (are these systems automated refill?) for the operations folks to follow, these type of seals work very well in harsh conditions.

 

[dabluffrat]

In my experience you won't see a plan 53 installed to performance test a pump. Check that the test setup will even be comparable to site conditions. You're likely testing with water, possibly even off rated speed and I doubt suction pressure will be as high as site conditions.... you won't be comparing apples to apples.

Did you know that 76.4% of all statistics are made up...

 

[JoeySoap]

Thanks for the thoughts, it's all useful information for me.