Increasing condenser water pump head 1

[ChasBean1]

Hello all -

Would like to know your thoughts on the following problem.

Putting it as concisely as possible, a chiller plant was replaced with a larger one. The condenser water system, however, re-used the old 6-inch piping to and from the cooling tower due to the cost of digging up piping and replacing it with 8" or 10". There's about 400 feet of pipe with a few turns and the velocity is high (~11 feet per second at design flow). The condenser pump suction, which is about 20 feet below the cooling tower sump (about 400 feet in x, -20 feet in y), reads about 16 inches of mercury below atmosphere when operating. The air separator even operates at a vacuum so (I believe) it does not have a way to expell the air it separates. The system periodically shows evidence of vapor binding and pump cavitation.

The best solution would obviously be to replace the piping. The cheapest solution I envision is a bladder type expansion tank piped into the top of the air separator, or at some other point on the pump suction line, near the pump. This would be located some 400 feet of pipe downstream of the cooling tower sump, and yes, in an open system. IS THIS REASONABLE? The thought is that some amount of NPSH would be added during operation, allowing the air separator to remove air while increasing the NPSH. When the system is shut down, the bladder should contract (or expand and push up the sump level?), but the sump volume rise or fall would be limited by the bladder size.

Is there a conceptual error with applying this to an open system? Has anyone applied a bladder tank in a similar instance to resolve such a problem?

Thanks in advance for any help with this. -CB

 

[steveen]

What is the temperature of the water to the pump?

The water is saturated with air and the frictional loss to the pump is so high that at the pump suction is 16" of mercury vacuum.

You are most likely cavitating the pump due to lack of NPSH available and the evolution of air from the water.

Without the water temperature, and elevation change from the height of the water in the basin it is impossible to determine what is causing your problems.

Consider a booster pump in the basin of the cooling tower, you can use the same piping and a low head high flow pump.

 

[KenRad]

CB,

With the expansion tank, I believe that the diaphragm would expand to its limit (or in the case of a bladder, fully compress), push out the water volume, and the result would be no change in pump suction pressure. I think I can prove this, but for right now, it’s just my gut feeling. The reason that the expansion tank works in a closed system is that the rest of the system is essentially incompressible.

Assuming the pumps you are now using are adequately sized (notwithstanding any cavitation problems you are having), what about moving the pumps closer to the tower?

Have you calculated the NPSH available and compared it to the NPSH required on the pump curve? That would differentiate between cavitation and entrained air.

If entrained air is determined to be the problem, what about a small vacuum chamber and vacuum pump connected to the automatic air vent valve? This should make the air separator work even though it is operating in a vacuum.

I would also check the pressure drop chart for the air separator at your new flow. You might be off the chart if it is undersized. If it has a strainer, you could eliminate some drop by pulling it out, assuming that the tower has some kind of strainer also.

---KenRad

 

[ChasBean1]

Thanks, guys. KenRad, I like the idea of using a vacuum pump. For the condenser pump, we found that the NPSHA is 15 feet and the NPSHR is 8.4 feet, so cavitation should be prevented although there may be fluctuations when the NPSHA drops (not checked).

There was a bit of a vortex in the cooling tower sump down the the pump suction inlet pipe, but it really didn't look like enough to draw in any significant amout of air...

Any thoughts on using a priming line (e.g., a one-inch line from the pump outlet to the pump inlet, or maybe to the top of the A/S) as an alternative?

Thanks again, -CB

 

[quark]

Kenrad is right that a bladder type expansion tank will be of no use, for this is an open system.

I did some rough calculations with 420 ft. of pipe, two elbows, a valve and a strainer in the suction line and getting around 11 meters of pressure drop. So NPSHa is around 17 ft(actually I ignored water level in the cooling tower sump). If the pump's NPSHr is 8.4 ft and still it is cavitating, you may be running the pump to the right of design point and flowrate may be more.

You can use vacuum pump for priming but when used during normal operation, this would further reduce the NPSHa. Booster pump as suggested by Steveen seems to be a better option.

One of the past threads indicates minimum submergence required to avoid vortexing, by the following formula

S = D + [(0.574Q)/D^1.5]
Where, S = Minimum submergence required in inches
D = Suction bell diameter in inches
Q = Flow rate in USGPM

Otherwise, try installin vortex breakers.

Regards,

 

[KenRad]

CB,

If you have room to move on your pump curve, I think that your small recirc line idea might work to boost suction head. This would be a good problem to run on a piping system software like Pipe-Flo.

quark - the vacuum chamber/pump I suggested would connect to the outlet of an automatic vent, which currently would not be able to vent the air from the air separator due to the subatmospheric conditions inside it. That is, it would only be connected to the system when the auto air vent opened due to an accumulation of air.

CB - this brings up a question - is there an auto air vent on the air separator now? If so, it is probably pulling air into the system instead of venting it.

---KenRad

 

[quark]

Ken!

Just think over, about the vent thing again. That is really not possible.

Regards,

 

[itdepends]

Ken

A recirc line to the suction of the pump will not boost NPSH. The pressure at the pump suction is a result of the hyraulic gradient needed to get the required flow from the source to the pump suction. If sufficient recirc flow was added to increase the pump suction pressure it would reduce the flow from the cooling water sump (hence defeating the purpose of the pump in the first place)


Regards,

 

[KenRad]

CB,

My curiosity got the best of me, and I ran a simulation based on a system and pump curve that is probably comparable to yours. What I found is that your recirc idea is sound in principle, but probably not worth the trouble.

In my example system, with a design flow of about 1020 GPM, you could increase NPSHA by about 3 feet by recirculating but you would have to recirc about 300 GPM to do it. So you would have to have a lot of room on your pump curve for the increase. The flow through your chiller would drop about 6% also, which may or may not be acceptable.

---KenRad

 

[25362]

There is also the possibility that the 6-in old pipe is heavily fouled up with (bulbous) deposits of corrosion products (biological or metallurgical) increasing the friction drop as estimated by quark for a commercial steel pipe. In this case the NPSHa would be lower than estimated and pump suction troubles at increased flow rates could be expected.

 

[ChasBean1]

Thanks for all your responses. We've laid out a few options for the client, none of which include addition of bladder tanks or recirc lines. Trying to think of a cheap "out" but band-aids like this will probably have us back there within a year or two... First and foremost (which they'll hate) is to dig up and replace the piping. The sump booster pump might be cheaper 1st, but we're still paying for a lot of friction loss in the long haul.

Thanks again, -CB

 

[CARF]

http://www.chengfluid.com/papers and articles/improve condenser.pdf

It's an article how to improve condenswater flow.

 

[quark]

How about laying an additional pipeline in parallel and joining both the lines at pump suction?

 

[steveen]

chasbean1,

Your comment of vortex not being a problem is not entirely correct.

The presence of a column of air in water (the vortex) will quickly drop the average weight of the column of water (over the piping inlet.) Hence, NPSHa will drop quickly when the vortex extends down into the basin. The vortex does not have to extend all the way down to the suction line to cause big problems.

Best to eliminate the vortex.
Steve