Problem with deadheaded Gardner Denver 10x12 centrifugal pump 1

[Kazah]

I am facing a Problem with deadheaded Gardner Denver 10x12 centrifugal pump.

Scenery;
We have multiple ports suction manifold (12-4" nipples) right in front of the pump. Two supply tanks are connected to manifold on the opposite sides of it. Tanks have the same water head. In normal operations, we are evenly lowing level inside the tanks. When valve on pump discharge side are closed (deadhead our pump) flow changes. We can observe flow of 0.5m3/min from one supply tank to another (filling the closest to the pump). Again, both tanks are on the suction side of the pump.
Please let me know if anybody has any suggestions or experience.

 

[BigInch]

put a valve in the suction side of the pump or on the tank outlets.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Kazah]

We have the valves on all inlets.
I described situation with fully open valves.
Does anybody know what the cause of it is?

 

[Scipio]

How long is the effect observed for and what approximate difference in heads does it stop at? If the discharge valve is closed the pump isn't drawing anything out of the tanks (kinda hard to tell, I'm not 100% about your layout, but I can't see any prerotation having enough of an effect on the tanks to keep things moving), but I'm guessing it'll be some kind of transient effect as the system returns to equilibrium.

 

[MikeHalloran]

With the pump deadheaded, it has to induce some rotation in the suction manifold. No telling what sort of weirdness that will cause.

Why on earth are you running the pump deadheaded?



Mike Halloran
Pembroke Pines, FL, USA

 

[pennpiper]

Here is my theory.
When in full operation the pump is taking suction from the tank that offers the least friction loss in the suction line. Then when you "Dead Head" (shut off) the pump, the tanks equalize their liquid levels.
To test this, close the valve at one of the tanks, Run the pump for a while. now shut off the pump and open the valve. You should see the same effect as before.

 

[ApC2Kp]

kazah,

Does the pump have a seal flush stream that is backing into the suction line to flow to tank? The 0.5 m3/min observed flow would be more than typical seal flush volume.

There might be transient effects to cause flow to go to the one tank. Coriolis acceleration (example - the rotation in the outlet flow of toilet bowl) might explain this, but then what do you want to change to stop the behavior? You mention the closest tank is increasing in level. The tank that is farther away has longer length pipe, hence more fluid mass. Inertia could force more fluid into the nearby tank until dynamic and static forces are in balance, then flow will reverse and possibly 'slosh' back and forth until the system finally settles into equilibrium.

 

[Kazah]

We do not have seal flush line going to suction line.
We could not wait until system settle into equilibrium because our closest tank became overfill (tanks have 40 m3 capacity each). Set up of the tanks very similar. Tanks have same elevation, same resistance. Also, I should specify irregularity happens only with Garden Denver 10-12 pumps.
I agree with ApC2Kp it can be Coriolis Effect, pump is powerful to develop it, but flow at 0.5m3/min and direction of the flow not usual for it. (as far as I know).

 

[vpl]

Do you have check valves in the tank discharge lines to prevent backflow?



Patricia Lougheed

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

 

[ApC2Kp]

kazah,

I could understand the pump being shutoff deadhead, while other pumps are switched to change system flow or destination to other points. I wonder if there is actually a level control problem, if a supply tank is overflowing. Is there any inlet flow to the two tanks when the pump discharge is shutoff? Could a level transmitter be out of calibration? Could a level control valve leak thru with rate of 0.5 m3/min ? Are there shutoff valves in the suction piping? Could a checkvalve in the tank discharge line (as mentioned by vpl) not prevent backflow into the tank that is overflowing, from failed checkvalve ?

The seal flush is mentioned since a pump with a single face mechanical seal could have external flush supplied, that typically would go thru a throttle bushing on shaft and into the pump casing. The throttle bushing would restrict the flush flow to minimize the amount of flush fluid that would other wise dilute the product fluid going thru the pump. During the discharge shutoff, the seal flush external fluid could only go out by way of the internal throttle bushing and pump casing to the pump suction piping.

An extreme off-the-wall sequence of events could result from the pump running deadhead for extended time. The pump will vaporize fluid (water to steam) with the horsepower input. The steam bubbles would flow backwards thru suction piping and up into the nearest tank. Steam bubbles in the vertical section of piping to tank would reduce the apparent density of liquid - it would act as an air column pump to draw liquid up into the tank. There is enough liquid in the tank to condense the steam bubbles and not over-pressure the tank. If other smaller pumps are deadheaded, their small horsepower doesn't generate the volume of steam bubbles that result in the 'air column' pump effect in the tank discharge piping.

Try turning off the motor when pump discharge is shutoff.

 

[Kazah]

We have only one pump. Tanks are storage open to atmosphere type without any supply lines. We do not have any check valves on the lines from tanks to suction manifold. All lines are horizontal. I like both ideas (seal flush and air column). But I have some questions: Do you think seal flush can generate internal circulation inside manifold (do not forget tanks are connected to the same manifold just in opposite sides of it), and second did you consider what in moment of evaporation we would lose suction and again we have to have internal (inside deadheaded pump and suction manifold) circulation.
We do not trying to prevented, we are trying to understand mechanics of it.

 

[ccfowler]

Mike stated the salient question best. Why deliberately deadhead a pump? The first thing to do is clean up that mess and then see about getting the rest of the system operating better, if any problems continue.

 

[Kazah]

We have to deadhead the pump (physical and technical limitation of the highway units).
We do it only one time during operation cycle.

 

[rzrbk]

If the 4" inlets into the suction header are off-center (hillside) or have some sort of internal projections, that would help explain how fluid rotation in the suction line develops enough head to overfill a tank.

If pre-rotation of the fluid in the suction is the problem, then a quick test/fix might be to install a temporary cone strainer right at the pump. This would interfere with and help break up rotation.

 

[RossABQ]

Install a recirculating flow line from the discharge piping back to the tanks, with a valve that opens when pump pressure exceeds normal conditions (i.e., when it is deadheaded). Any pump this big should not be routinely run deadheaded. A 1.5" line should be enough.