IMO Pumps in Parallel 1

[jgkg]

We use a piece of equipment with an oil system that has two pumps, one is a shaft driven oil pump (IMO C3E3B-143) and the other is a motor driven pump (IMO A3EB-87).

The shaft driven oil pump is a capacity of approx. 8 gpm at 12 psig (we use Mobil 797 oil). Approx. 4 gpm go to the pump bearings and the rest is recirculated back to the oil reservoir set at 12 psig.

Each pump has a check valve.

The procedure for starting the equipment is the motor driven oil pump is started and the equipment is started if the oil system pressure remains above 5 psig. Once the equipment is started, the motor driven oil pump is stopped.
If the oil pressure drops below 5 psig, the motor driven oil pump will cycle.

When both oil pumps are running, the relief valve is assumed to be recirculating everything in excess of 4 gpm.

Here is the question. Assuming both oil pumps are matched in capacity, will the oil system pressure momementarily be cut in half until the relief valve can compensate, when one pump is shut down?

 

[JohnGP]

If all is operating as it should, then when one pump is shut down the flow through the relief valve should reduce, but not the system pressure. I'm not familiar with the pumps you mention, but anyway, the relief valve should maintain the system at 12 psig whether one or two pumps are running.

If the oil pressure is dropping below 5 psig then that suggests to me that either the shaft driven pump cannot perform to design, or more than 4 gpm is going to the bearings, or there is a problem with the relief valve.

Just a few of my thoughts,
Cheers,
John

 

[vpl]

Define "momentarily." While there is going to be some transient behavior, which might include a transient pressure decrease, there isn't enough information provided about the response characteristics of the system, either pump, the relief valve, or the controls to define whether the pressure drops in half.

One way to tell is if your relief valve chatters when the motor driven pump turns off. If it does, it's at least telling you that pressure is dropping enough to hit the reset setpoint (whatever value that is.)

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[JJPellin]

This is a standard configuration for many lube oil systems. The relief valve must be a full-flow relief valve, not a "pop" valve. It is really more of a pressure regulating valve. It does not have a fixed set-point. It has a pressure at which it cracks off of the seat. But, as the flow through the relief valve increases, so does the pressure. A pressure transient is common in a poorly designed system. You don't mention any other pressure regulator, so I assume that the relief valve is the only pressure control. In this situation, we would suggest adding a bladder accumulator. This will smooth out the transients when the auxiliary pump starts and stops.

In some of these systems, they deliberately set the relief valves on the two pumps differently. If the pumps and relief valves were identical, you have a problem with the auto-start. The auxiliary oil pump would start up and satisfy the set-point to shut itself back off. This occurs over and over until the motor trips off and locks you out. Either, the auto-start switch needs to be a latching relay that will not shut back off automatically, or the two pumps have to produce different pressures. We have systems where the main pump relief is set to 20 psi and the auxiliary pump relief is set to 10 psi. The auto-start switch is set to 15 psi and is not a latching relay. The auxiliary pump starts and stops automatically when the main machine comes up and down. But, for most systems, we have the two pumps set identically and the relay set to latch. Once the main pump is up and running, an operator has to switch the auxiliary pump from, “Auto” to “Off” and them back to “Auto”.


Johnny Pellin

 

[TenPenny]

Remember that the IMO pumps are 3 screw pumps, and essentially each will pump at 8 gpm no matter what pressure (within limits, of course).