Recirculation in Centrifugal Pumps 1

[Lijantropo]

Good morning,

I have a process that needs a constant level in a tank. For this, there is a proportional control valve on the enter line. My question is about the pump that we use to pump the oil:

When the liquid level in the tank reach the established level, the control valve will stop the flow for a while. In this point, the pressure in the discharge line increases and the capacity (gpm) decreases. May this cause any kind of damage to the pump? Should I have a external recirculation line to avoid internal recirculation due to this suddenly stop?

Thanks,
Lij.

 

[SNORGY]

I would.

Set / size the control valve to recycle a flow equal to or just greater than the minimum flow established / specified by the pump vendor. Your intent is to mitigate:

(1) power and heat added to the stalled fluid, driving up temperature at the seal faces
(2) unbalanced forces on impeller(s) due to operating significantly left of BEP, giving rise to shaft deflection and accelerated bearing and seal wear

Regards,

SNORGY.

 

[TD2K]

Centrifugal pumps do not like to run at no flow or below the manufacturer's recommended minimum.

Depending how the filling cycle is expected to occur you may want to consider a minimum flow line or a auto start/stop on the pump. Products being loaded on trucks in bulk distribution terminals for distribution are often automatically started and stopped as needed.

 

[skearse]

Yep. I'd either add a 'spillback' or recirc valve, or an interlock to shut down the pump [shut off would probably be my preferred option-why waste the pump energy if you don't have any 'real' work for it to do. There may be other process considerations, but that's what I'd work towards.]

 

[BigInch]

Yes, this is a total waste of pump energy and a control valve. Use one hi level control, run the pump at a nice BEP speed and turn it off when it reaches the hi limit.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Lijantropo]

Thank you all for your responses.

The tank have to maintain one specific level all the time: I can not work between two levels so I can not use a level switch to shut down the pump.

Usually, the pump will work at normal capacity, but my question is related to the problems if the consumption in the tank is stopped for a moment -due to any kind of situation- and the pump have to work in minimum flow or below.

In this case, I think is better to have a recycle -to avoid to stop the pump-and wait for normal capacity.

Regards,
Lij.

 

[TD2K]

When you say a constant level, how much variation can you accept? A tank is usually there to provide surge capacity, there's no surge capacity if the level can't change.

Even if the level can't change much, depending on the requirements, a foot level change 'could' be a significant volume with minimal impact in terms of maintaining 'constant' level. It all depends on the tank size and the outflow requirements.

I'm just trying to suggest you may want to ensure that some of the constraints you are dealing with (the tank level 'must' be constant) have a sound engineering basis behind them. Obviously this is very general because of the lack of details we've been given here.

 

[BigInch]

Yes exactly. That method of operation defeats the whole purpose of having built that tank.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Lijantropo]

Good Morning,

The tank was built for ensure at least 10 hours of supply in case of problems. This kind of operation (constant level) was a client's request so I have to operate that tank in this way, improving the operation of all the other equipments. The tank size made that "the 10 hours-level" is around three feet under the roof level. I mean, two levels will imply to work very close to roof, and this is undesirable because this reduces the capacity of the tank to catch overflows.

Due to this constrain, my attention is focused on the centrifugal pumps.

Regards,
Lij.

 

[BigInch]

Why not charge the tank, then bypass until that supply is needed?

I didn't say two levels, just a high level. One high level control does what you want, no?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Lijantropo]

Good Morning,

Mr BigInch: I was thinking on start/stop the pumps, and that is why I thought in two levels: Low-level to start, High-Level to stop.

I am going to analyze the bypass operation.

Regards,

Lij

 

[BigInch]

I think as long as you didn't need to keep the fluid in the tank fresh, hot, cold, or circulating for some reason, it might be a good idea to try it. Apparently your process doesn't vary, so the tank is just a supply security thing.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[moondog]

You could place another val off the discharge of the pump and cascade control from the first val. Val 1 at 80% the new val. val. 2 at 20% val. 1 at 100% val.2 at 0% this would keep the same flow at all times.