4 replies to this topic

[BrianJR]

Hi all,

 

I was wondering if you knowledgeable folks would mind having a look at the attached picture of our pumping system and commenting on this setup.  Please note that I don't have an engineering background, so I'm trying to bring myself up to speed on many terms that you folks probably use on a daily basis! lol

 

I would specifically appreciate comments on whether or not this is a good setup for a bypass valve on a centrifugal pump.  This system is used to drain a large wave tank containing seawater.  We need to drain the tank at a specific rate (60gpm), so the gate valve is used to vary the flow until we reach the desired rate (not shown is a flow meter on the left).

 

When the gate valve is fully open, the system pumps at ~70gpm.  We close the valve until we get our desired flow rate of 60gpm.  During this time, the valve on the bypass is always kept closed.  Would it be better to adjust the flow by keeping the gate valve fully open and varying the flow by opening the bypass valve?

 

Also, this system is being modified later this fall so that we can pump at much higher rates.  We are going from a 2hp pump to a 20hp pump, and from 2" PVC to 4" PVC.  Our plan was to simply scale up the current design for a bypass valve, unless anyone has any suggestions for ways to do it better.  One change with the new system is that we will be varying the flow rate (as low as 120gpm to as high as hopefully 500gpm), so I'm assuming the bypass valve will have to be opened to prevent the pump from going below its minimum flow.

 

Thanks in advance!

 

 

Attached Files

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[Steve Hall]

You should keep the bypass valve closed with your current setup unless the pump is dead-headed (briefly dead-heading the pump is OK).

 

Minimum flow is pump-specific; your pump vendor will help you determine the actual value. Minimum flow may be based on heat build-up in the pump or stress/vibration. However, at 500 gpm if the pump is operating near its best efficiency point (BEP) then you can probably throttle the discharge to achieve 120 gpm without worrying about a bypass.

 

If you think a bypass is required, then consider using a restriction orifice (RO) instead of a valve. The orifice would be installed in place of the valve on your bypass pipe. It would be sized so the flow through the pump, when the gate valve is fully closed, would equal the minimum flow for the pump. There would always be flow through this line, but when you open the gate valve the majority of it would flow to your destination (opening the gate valve will decrease the pressure at the pump discharge and reduce the amount of flow going through the bypass).

 

You might get better results by varying the pump speed instead of using a throttling valve. This would require a Variable-Frequency Drive (VFD) and control loop. So there is a cost - probably $10 to $15K.

[BrianJR]

Thanks for the response Steve!  Could you explain what you mean by "throttle the discharge to achieve 120 gpm without a bypass"?  Do you mean simply closing the gate valve to achieve the flow required?  I don't know the minimum flow rating on the new pump, but you are right that 500 gpm is near the BEP.

 

I like the idea of the variable-frequency drive.  Can it be installed on any pump?  We have already ordered the new pumps, so should I check with the manufacturer to see if these motors can be run with a VFD?  

[Steve Hall]

You do have to confirm that the pump motors are compatible with VFD, but nearly every modern motor can handle it even if not specifically stated in the literature.

 

And yes, simply close the gate valve to achieve the flow required. Isn't that what you're doing now to achieve 60 gpm? The downside is that gate valves aren't usually considered very good at throttling - for your bigger pump you might have to close it pretty far to reduce the flow from 500 gpm to 120 gpm and this will result in a very high velocity through the opening. High velocity can mean noise, vibration and reduced valve life. Precise flow control would normally be achieved with a globe valve or possibly a butterfly valve. But you can try it with the gate.

 

Did someone do hydraulic calculations to be sure your components (pump, pipe size and control valve) are all sized properly for your flow rates? They should also know about the minimum flow through the pump.

[BrianJR]

Woops, I meant to say globe valve, not gate valve.  So yes, the new system will use globe valves.

 

We have an engineering contractor who has helped us with this build (spec the pumps, pipe flow calculations, and design of a manifold).  But as far as the pump configuration, he basically said to just use the same design as we currently have.  So before approaching him with these questions, I just wanted to sound a little more knowledgeable.  So thanks for your help!  The suggestion of a restriction orifice and VFD are good ideas and I'm looking forward to seeing what he has to say.  Cheers.