TDH calc question

[krugan]

I have the following scenario where I am trying to find the proper way to do a TDH calculation for a pump when it is in reversed direction with suction from a common header.

I have four tanks each with their own reversible rotary lobe pump with a 10" pipe on the inlet & outlet of the pump. The 10" outlet from each pump connects to a 20" common header which flows out to a clearwell.

Each pump is producing about 800 GPM into the common header and during certain periods each pump takes a turn to reverse the flow.

My question is, how should I calculate the TDH for the pump when it is reversing the flow and drawing from the common header? Should I use the min and max height of the clearwell level to do this and ignore the pressure from the other pumps, or do I treat it as one of the other pumps is acting like a feed pump?

 

[Artisi]

the total head across the pump will be the static head on the pump discharge plus any friction or other losses on the delivery side minus the inlet pressure. The inlet pressure is the head generated by the pumps pumping into header, not the height of the clearwell level.

 

[packdad]

I'm not very familiar with rotary pumps, but wouldn't the TDH be the same regardless of direction of rotation, as long as the speed is the same in both directions? It seems like it would, but like I said - I'm just assuming.

I guess you don't have suction and discharge pressure gauges installed? That would make it very easy!

 

[krugan]

Thanks for the reply Artisi. Packdad, for this case the static elevations are different. When pumping from the tank the static elevation of the tank is 16', pumping to a clearwell via a common header that has a static head of 3'.

When the pump reverses and pumps into the tank, it is drawing from the common header. I wasn't sure if I needed to take into account the static pressure from the clearwell somehow.

When flows are merging in a header, are there frictional losses to account for?

 

[FiltrationEngineer]

Isn't a rotary lobe pump a positive displacement pump? If so, usually TDH isn't much of an issue as long as the motor can handle the load.

In any case, the TDH of the pump is the pressure on the outlet side of the pump (either direction) - the pressure on the inlet side.

Is this for a membrane system?

 

[sailoday28]

Artisi (Mechanical)For the TDH, you should include the change in V^2/2g and elevation, between the discharge and suction nozzles of the pump.\

Regards

 

[KernOily]

TDH = total dynamic head = the energy that must be supplied by the pump to do the required job, over and above what you already have present at the suction.

In my experience, the V^2/2g term can usually safely be neglected because it is very small compared to the other terms in the energy equation. Your mileage may vary though. It's a quick check to see if you need to include it.

Why do you need to know the TDH? Is the motor not big enough? Surely your new discharge pressure is not in excess of the MAWP of the pump? Or maybe you need to verify the setting of the PSV on the discharge? (I assume you have a PSV since this is a positive displacement pump...).

Thanks!
Pete

 

[sailoday28]

KernOily (Petroleum) 20 Mar 07 17:30
TDH = total dynamic head = the energy that must be supplied by the pump to do the required job, over and above what you already have present at the suction.
Correct me if I'm wrong, but doesn't the energy supplied to the pump depend upon its efficiency?

Regards