Low pressure hydrualic accumulator question

[byrdj]

I am thinking about how to improve the realibilty of a lube oil system during a system failure of loss of one pump and the autostart of the back up pump. the system has 2, dual output pumps, of 200/60psi. the system flow rates are about 10/100 gpm. testing, the time from running pump stop to standby at pressure is about 2 seconds, with pressures dipping to 60/30 psi

would piston type accumulators be usable at these low pressures?

 

[MikeHalloran]

If the standby pump comes 'up' before the pressure hits zero, an accumulator wouldn't provide a benefit.

Adding components can only reduce the reliability of the system.

Additionally, piston type accumulators and pistons have been known to stick. At the stated range of system pressure, a bladder type accumulator might be a better choice, if an accumulator's presence makes you feel better.



Mike Halloran
Pembroke Pines, FL, USA

 

[byrdj]

I would think that the accumulator would reduce the rate of the pressure dip while the standby is coming up to pressure.

this lube oil system is on a critical component and having oil pressure dip will cause an auto stop. the current dip is right at the limit, but that is utilizing the flow from the stoped pump's coast down.

 

[MikeHalloran]

You didn't mention the auto stop before.
Of course an accumulator would give you a little margin above its trip point.

The accumulator might also delay transition of the rising oil pressure past the trip point on startup. There is presumably a timed gate that inhibits shutdown during startup that might need adjustment to allow time to charge the accumulator. ... and any modification to that might upset some other alarm system, and so forth.

SO, you should consult the manufacturer of your critical component, whatever it is, before hacking any part of it.




Mike Halloran
Pembroke Pines, FL, USA

 

[Oldhydroman]

Do the working pressures you indicate arise from the fact that the pumps are delivering more oil than each system requires and the surplus is spilling over a relief valve in each circuit?

If so, then you might be able to overcome the start-up delay by adding some externally drained sequence valves to your circuit. Correctly set up these valves will allow the system pressure to rise as before and only the surplus oil will be used to charge the accumulators. You would still need to retain a relief valve so the surplus oil has somewhere to go when the accumulator in that circuit is full.

I agree that bladder accumulators (or diaphragm accumulators) would be a better choice than piston accumulators at these relatively low pressures.

DOL

 

[byrdj]

the pumps are submerged multiple stage centrificals, with the 60# header off of the first stage. there is no relief or spill off.

So bladder type accumulators can be used at this low of a pressure?

I was more familiar with higher pressure (2000 psig) piston type accumulators being used to provide quick flow to supliment when demand exceeded pump capibility. like for the 60# header, I would be needing about 2 gallons of fluid discharge during the transient.

this lube oil system is for a steam turbine (about 10MWs size). I have contended that this design is adaquate to provide protection of minimun flow to the bearings with a running pump failure, but it is marginal if it will prevent a trip on low bearing pressure. thes turbines are used in a process where tripping would challange critical safety systems in the plant

 

[LittleInch]

In order to limit the size of your accumulator you may need to use a much higher pressure accumulator fed by a small separate high pressure pump and pressure regualting valve set to open at below your normal operating pressure but higher than your min pressure so that at the end of the accumulator capacity (3+ gallons)you still have > 60 / 200 psi available. To have one which only uses the current working pressure will require a large unit so that it doesn't run out of pressure if it's only starting at 60 / 200 psi and note the issues with diversion of flow during start-up of the back up pump above.

Given it's apparent vital nature, has any one considered running two pumps normally at lower capacity with a third on stand by? might still be cheaper than the accumulator route?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[byrdj]

ever since this oil pumping arrangement was installed +15 years ago, the norm has been to run both AC oil pumps.

I have cut & pasted a schematic of the current system and added my accumulator thoughts in red.

Thanks for all the suggestions as this is one of my peeves I would like to solve

 

[LittleInch]

The problem with your design as I see it is that the accumaltor either has to be very big or the pressure decays very rapidly. I can't see the point of the orifice plate - all this will do is prevent fluid flow into the pipe and you need flow very quickly.

An alternative may be a nitrogen bottle plugged into the other side of your cylinder set at say 50 / 170 psi and a relief valve set at 60/ 200 to blow N2 back out when it re-fills. This would reduce the size of your accumulator, but you'd need to find room for the N2 bottle, but one bottle could feed both systems so long as you sized the regualtor correctly. Of course you then ened to make sure you replace the bottle every now and then....

However if you run both pumps at the same time what's your peeve with this?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[byrdj]

the orifice is to restrict the refill. the loss of pump/standby pump autostart transient is only a couple seconds. I would want as much of the starting pump capicity going to reestablish flow to the unit and not going to refill the accumulators.

the check valve direction will allow rapid accumlator discharge into the headers. the total flow for the 60# header during this 2 seconds is about 2 gallons, with the 200# header only about 0.2 gallons. a 10 gallon accumulator charged to 30 psig is about what I was playing with for the 60# header. Actually, when I get some time, I want to do spring calculation to see if that could be possible.

Operating with dual 100% has caused some problems with one pump dead heading the other. this was band aided by addind a small venting flow to each MOP.

during the 15 years this configuration has been in service, there have been several lose of MOP events. while the dual operation was being performed, things were ok, except for having to make the decission to operate with just one while the other is being repaired. But as Murphey has it, a couple of the loss of MOP happen when the performance of the system was blessed as acceptable, but it wasn't at the time of the event

The current mode of runnig with dual would be the cheapest, since replacing both pumps every 4 years would be less than the paper work to do something else.

 

[LittleInch]

Sorry, misread the way the NRV was pointing. I'd still go for the bladder option rather than the spring and piston due to the potential sticking issue.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[byrdj]

sorry for the inconsistant in component designiations, I was cut and paste ing from the existing plant drawing.

Thanks for the comments.

 

[stgrme]

Perhaps, you have already considered the following action. Why not raise the set point on the pressure switch used to start the back-up pump? Starting the back-up pump earlier will decrease the amount the pressure falls during the changeover.

Best of luck!

 

[byrdj]

The recomended setting is as high as possible taking into account deviations between different units. The pump start logic pressure switches PS4s are set to "reset" 10psi below the lowest expected header pressure (190). the PS dead band is 20 psi, so the start should occur at 160 decreasing. During the years, there has been times when both pumps running was needed to bump the header high enough to "reset" the autostart PS4s.

I have heard from another owner, where the OEM provided them with accumulators for this concern. However, I have not been able to get any details other than "they look like scuba tanks" and "it worked". My customer stated they will submit a request to the OEM.

 

[byrdj]

Stubled accross an OEM sales publication for one of thier other turbine divisions for low pressure accumulators were available back in 2002

http://www.ge-energy.com/content/multimedia/_files/downloads/dataform_2053435705_2809808.pdf