3 replies to this topic

[Pronab]

By-Pass valve arrangement

By-pass Valve Around A NRV Of A Pump Discharge Line

Mar 24 2006, 10:08 AM
Post #1



In a small pump arrangement (LT motors); we have always a stand –by pump to give a facility: auto start in case of the running pumps failure.
Discharge line of the pump contains a NRV (non return valve) followed by a globe valve. And there is a small by-pass valve around the NRV and the globe valve. During normal operations of the pump, this small by-pass valve kept close with blinded, as showing in the P&ID. This by-pass valve is not for equalizing the discharge line as pressure difference between suction and discharge is very low (from 4 to 6 barg). But I don’t understand why this small valve should be in closed condition rather than crack open to give a little flow from the running pump so that stand-by pump casing and piping’s will be in warm condition at all the time, keep ready in case of the running pump fail. (Suction fluid temperature of the pumps vary from 55 to 110 deg. C.)
Is my idea correct? Any comments are highly appreciated.

Best Regards.


Mar 24 2006, 01:34 PM
Post #2
By mistake it appears on the STUDENT page, it should be on professional item.
Pronab.

Art Montemayor


For a while there, I thought an old Pro like you had gone back to University when I saw this thread in the Student Forum. I'll give some comments here because I believe this is a great, practical subject for all student engineers to become familiar with and study the application.

From what you describe, I had to generate a quick Excel sketch in order to make sure I know what centrifugal pump arrangement you have. I assume your have centrifugal pumps.

From the attached sketch, I am assuming that I have depicted your installation correctly; if not, please correct me. I believe the small, by-pass valve(s) that are around the check valve and the discharge block valve only serve to act as manual priming devices for the stand-by pump or the running pump. The priming fluid would be the fluid left over in the discharge line. Otherwise, I see no need for the by-pass valve.

Because you haven't identified it, I have had to assume the operation of putting the stand-by pump into operation (while shutting down the running pump) would be manually done. That would explain the by-pass valve. Under the illustrated piping configuration you can see that the check valve is preventing any self priming by the pump from the discharge side. There are various ways to manually prime the stand-by pump and one of the simplest ways is to vent the top of the casing (usually back to the suction tank vapor space – not shown in my sketch) and open the by-pass valve to allow discharge fluid to flow through the casing and back to the suction tank. Once

the casing is full of liquid, the by-pass and the vent valve are closed and the pump’s suction block valve is opened. The pump can now be started under primed conditions and should pump immediately.

If you have totally instrumented and automatic start-up of the stand-by centrifugal pump, then you require automatic-controlled by-pass valves operation as well as the inlet and discharge valves. Otherwise, the system won’t shut down one pump and startup the other one. By the way, what is an “LT” motor?

I can understand your concern for trying to keep the stand-by pump warm while it is sitting there idle, but from what you describe, the by-pass valve contributes little or nothing to keeping the stand-by pump warm or hot. In order to keep the stand-by pump under process temperatures at all times, you need to trace the pump with steam or electrically on the outside surface of the casing. You can direct the discharge of one pump into the suction side of the stand-by and let the hot fluid discharge of the running unit go through the idle stand-by, but this will cause a pressure drop (beside spinning the impeller of the idle stand-by. The best, simplest way is to keep the stand-by pump heat traced at all times. This method avoids a lot of complicated piping arrangements and configurations. If you have jacketed pump casings (which is a high probability in the case of hot fluid pumping), then you can run the discharge of the running pump through the jacket of the stand-by pump.

You also have not explained if you are alternating the service of these pumps. For example, in the actual industrial case where it is highly important to have back-up pumping capacity at all times, the usual practice is to have the running pump assume the role of “stand-by” after it has been shut down and replaced by the stand-by unit. This implies that both pumps should be regarded as “stand-bys” and, therefore, should be piped and instrumented almost identically in every respect.

I hope these comments and response help to explain the installation.


Reply to Art Montemayor

 I acknowledge earlier, by-mistake it appears in the STUDENT FORUM instead of professional forum.
 You are right; I am talking about centrifugal pumps.
 Definitely it is not used for priming or draining the pump. For priming there are valves in the pump’s casing: from top and bottom. After priming we kept the pump as STAND-BY so that it will start in auto, in case of running pump trip (trip signal activate to start the stand-by pump).
 Stand-by pumps are not started manually, always start in auto, I mention earlier.
 LT motors mean low tension motor, where power supply is 415 V.
 For test case we keep this small by-pass valve little open, found casing of the pump become warm. If discharge line NRV passing, pump will start reverse rotation (by-pass line is small compare to discharge line size, so that it will prevent reverse rotation).
 Keeping this by-pass valve little open, running pump amp and discharge pressure change very little. It is not remarkable.
 We have pumps those having steam tracing, handling fluid which need a certain temperature to prevent solidification at low temperature. It is handling liquid sulfur.
 Yes we do. We change over these types of pumps to check its reliability at regular intervals.
 I can not open your attachment. Can you attach it again.


Thanks and best regards.

PRONAB.
March 30, 2006.

[Pronab]

Reply to Surath

This valve is not for draining. Because there is another valve for draining.
If NRV is passing of stand-by pump, in that case stand by-pump will rotate
reverse. Yes, the by-pass valve is very small in size to having a little contribution.
Please have look on the attach drawing.

regards.
Pronab.

Attached Files

•  by_pass_valve_arrangement.doc   34.5KB   86 downloads