Cooling Tower Make-up Pumps
Cooling Tower Make-up Pumps
(OP)
I'm having an issue at my plant with our cooling tower's make-up water pumps.
What we're seeing is our cooling tower calls for make-up water, solenoids valves open downstream of the pump, five seconds later our make-up pump turns on, and then instead of pushing water up to the tower, the pump appears to just spin, eventhough a pressure gauge near the outlet reads 160psi. The expansion tank is always full as designed and the fill control loop seems to work well on it.
Some times the system will work right away, other times it takes an hour or two of manually overriding the control system to get the fluid up to the tower.
Other system details,
- cooling tower is 250 ft above the pump outlets
- expansion tank's surface is about 6 ft above the pumps
- fluid is water at common city supply temperatures
- the system worked last summer without issue.
I've read in our troubleshooting manual that the pumps may need to be primed. An online search seemed to point in that direction too, but my operator is adament that they do not need to be primed and that it is not possible for air to be causing the problem. I'm not sure I agree, but I'm not a pump expert and would like to hear what other people think may be causing the problem.
As always, appreciate any feedback or thoughts.
What we're seeing is our cooling tower calls for make-up water, solenoids valves open downstream of the pump, five seconds later our make-up pump turns on, and then instead of pushing water up to the tower, the pump appears to just spin, eventhough a pressure gauge near the outlet reads 160psi. The expansion tank is always full as designed and the fill control loop seems to work well on it.
Some times the system will work right away, other times it takes an hour or two of manually overriding the control system to get the fluid up to the tower.
Other system details,
- cooling tower is 250 ft above the pump outlets
- expansion tank's surface is about 6 ft above the pumps
- fluid is water at common city supply temperatures
- the system worked last summer without issue.
I've read in our troubleshooting manual that the pumps may need to be primed. An online search seemed to point in that direction too, but my operator is adament that they do not need to be primed and that it is not possible for air to be causing the problem. I'm not sure I agree, but I'm not a pump expert and would like to hear what other people think may be causing the problem.
As always, appreciate any feedback or thoughts.





RE: Cooling Tower Make-up Pumps
The way you describe it the pumps have to overcome a 250 ft static lift at the outlet, equivalent to about 7.5 bar or about 109 psig. Presumably after the static lift it discharges into an open system.
Thus if you are measuring 160 psig, and the system was once working, I would suspect that you have a blockage or closed valve somewhere in the system.
Did you ever take pressure readings when the system was operating correctly ? If so what did they show ?
It sounds like you have a decently flooded suction (no suction lift), so really the only way for air to get into the pumps would be for the suction inlet to vortex from the free surface of the tank. However since you appear to be generating pressure at the pump outlet, I don't believe that is happening.
RE: Cooling Tower Make-up Pumps
You're correct, our system is open.
And we have also looked for any valves that are sticking, but the only valves that we can find appear to actual properly when the logic tells them. I base this on the sound they make, but perhaps they need to be completely taken apart.
Unfortunately, we do not have any pressure readings from when the system was functioning.
Thank you for your feedback.
RE: Cooling Tower Make-up Pumps
What controls are you having to work exactly? So far it does kinda point to a malfunctioning solinoid on a valve, or perhaps a roughly sheared valve stem that catches sometimes and operates the valve, but sometimes not.
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Cooling Tower Make-up Pumps
You might also want to check ALL of the distribution valves up on the hot water deck of the tower. Many of these are really crappy valves and although the stem may make it look like it is open, one of the primary valves may actually be stuck closed.
Also make sure you trace the discharge line completely. Many of the half*ss tower rebuilders put valves in the weirdest places.
With the 160psi pressure, your's does not sound like a pump problem.
RE: Cooling Tower Make-up Pumps
Perhaps you could further elaborate on your above statement what you did with the control system to get it working.
RE: Cooling Tower Make-up Pumps
RE: Cooling Tower Make-up Pumps
On butterfly valves, commonly the taper pins that retain the disc to the shaft can shear. The disc stays still while the actuator/shaft rotates, and can cause the intermittent opening. On the outside everything will look normal.
The valve will need to be taken out to check the internals, and also check the seats. Rubber seats may be pinched and deformed, causing the pins/shaft to shear in the first place.
Crack the manual drains/vents or check gauges along your pipework. Seeing where the deadheaded pump pressure drops off will indicate your problem valve. Check the pressure ratings of your valves as 160psi exceeds the lower classes.
RE: Cooling Tower Make-up Pumps
I'm not a pump expert and would be interested to hear anyone's thoughts on how appropriate a pump with such a curve is for this application.
I'd also like to ask if pipe systems such as this typically have check valves in them? Our system does not.
Thanks
RE: Cooling Tower Make-up Pumps
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Cooling Tower Make-up Pumps
Say for example your frictional head loss was very low, the pump would runout to 480 USGPM which is 130% of BEP. This is higher than preferred but acceptable on a smallish pump like this assuming NPSHa is adequate.
Since in reality you will have some frictional losses, it is likely the pump is operating closer to BEP. In which case I would say it looks fairly well matched.
RE: Cooling Tower Make-up Pumps
Certainly sounds like the pump is doing what is told to do and isn't the problem.
Sounds like you have traced the discharge line and there are no closed or obstructed valves??
Looking at the performance curve for this pump, I note that it is extremely flat; meaning that there is very little change in pressure as flow varies from shutoff out to design point (absolutely flat from 0 to 250 ft head).
You only have a 10-15 psi variance from shutoff to operating point (all of that is above 250gpm) and you are using pressure to try to control the pump's flow with an extremely narrow range of pressures to work with.
Imagine if the steering wheel on your car only had 1/8 turn to control the entire car; the smallest movement would cause you to swerve all over the road. OK, that is a cheesy analogy but is somewhat similar to what you seem to have.
A pump with a steeper curve would have been much better for this service, but I realize you are probably stuck with the one you have.
Your system curve is also somewhat flat as it is mostly static head (250ft). WIth just a little bit of line plugging from scale or obstructions, the friction portion of your curve will rise dramatically and will move the operating point of your pump back to the left (pump will ALWAYS run at intersection of the performance curve and the system curve).
Once the operating point moves left of the 250 gpm point, your pump will essentially "wander" back to shutoff and flow goes to zero.
This is pumps 101 stuff to most of the guys on this post, I'm putting it in because you said you were somewhat new to pumps. Maybe some of the other guys (or gals) could expound upon how your system curve is going to dictate where the pump operates. I would like to hear some opinions on how this could be solved given the above assumptions are correct.
RE: Cooling Tower Make-up Pumps
First the curve gives no NPSHr curve or value. Curious. It is a pump basic.
I notice that the suction velocity is horrendously high or at least it is in the pump world(s) I have lived in.
You state that the liquid level is 6 ft above the surface.
You state that the operation is intermittent. Does the water in the expansion tank (this is the suction source??? right???) remain at city water temperatures or does it rise (sunlight heated) while not pumping?
What is the duty cycle?
I suspect that your pump is cavitating. Can you hear any unusual noises associated with the pump operation, especially when it is not appearing to pump?
Is the expansion tank surface open to atmosphere? If so, can there have been any trash that has entered into the piping between the tank and the pump? (dead birds, pipe scale, etc?).
If closed, is it pressurized at all - I can't see where you mention that.
Even though you state that the expansion tank flll control works well, it is not instantaneous. Does the level pull down when the pump initially starts?
Where is the check valve that prevents emptying of the line between the pump and the CT? Is the discharge guage you are reading upsteam or downstream of the check valve?
What the heck size CT is it that wants 300 GPM instantaneously as makeup? You don't have to answer that question if it is personal - just curious.
I suspect that at a zero flow condition when your pump starts, before the flow losses begin to kick in, this pump runs out on its curve and cavitates the pump. If you can obtain a NPSHr curve for this pump, I believe that you will see it begin to rise steeply in the region that you are trying to operate.
And, if no check valve, and the system is depending on the solenoid valves to hold liquid in the line, then in the seconds before the pump starts, the line can be draining back into the flow tank, and emptying the line reducing the head pressure on the pump guaranteeing that it runs out on its curve.
That is my story and I'm sticking to it until someone pushes me off my soapbox.
rmw
RE: Cooling Tower Make-up Pumps
If the pump is cavitating why is it putting out 160 psig ? That value is basically the shutoff head of the pump. Even if the pressure gauge is after a check valve it should read 109 psig if the pump were failing from cavitation, not 160.
Also to design a 360 USGPM pump that required a flooded suction (33 ft NPSHr) gives you a Nss of 4200. 4200 is impossibly awful performance IMO.
All of which means I tend to disagree with it being cavitation unless the OP has omitted some detail.
RE: Cooling Tower Make-up Pumps
macmet, you have not feed back on the question about having to manually over ride the control system to get the water up to the CT.
I suspect that the solenoid valve did not open.
Without a check valve in the discharge piping means the water in the vertical pipe will drain back to the suction tank every when the pump stop. Unless the solenoid valve you mentioned is closed before the pump stop.
It it very strange to use a solenoid valve when the pump is solely supplying water to the CT
Is your solenoid valve located close to the pump or at the out let of the discharge pipe?
It would be better to install a check valve near the pump (with by-pass valve for draining) and configure the control to start the pump motor directly from the CT level switch.
RE: Cooling Tower Make-up Pumps
If discharge piping is either scaled up or obstructed by partially closed valve, your friction losses may have grown too large for the pump to overcome.
Your pump was designed to support about 350' total head; 250' static head plus 100' friction head. The maximum head this pump could ever support when it was new = 380' total. Too bad you already have max diameter impeller already installed.
If your friction head has surpassed 130' for whatever reason, this pump will not be able to get water up to the CT basin.
Pump wear also eats into the pump's ability to support the 350' required.
The elaborate control system seems unecessary; maybe just go with float valve level switch in the cold water basin to control pump's on/off??
You could put a common HVAC triple duty valve on discharge flange to take care of shutoff/check/balance duties??
RE: Cooling Tower Make-up Pumps
Pumpsonly... I guess I tried to explain the manual override of the system to get the water up to the towers. But I will try again.
Our towers have a water level sensor in the basin that has a high, regular, and low sensor. When the sensor gives the input to the control system that the water level is low, the system sends a signal to the solenoids to open, and five seconds later the pumps get a signal to turn on.
When this happens, you can hear the solenoids "open". What I mean by this is, is just that you can hear them click. The pumps when they turn on, begin to produce 160psi.
Sometimes this sequence succeeds in getting water up to the towers. Other times the pumps just produce 160psi but no water gets to the tower.
When we notice this, we put the low level sensor into override and toggle the input between low/not low. When we do this we can hear the solenoids opening/closing (clicking). Usually at some point the flow will all of a sudden start. Sometimes this takes 5 minutes other times a few hours. We have not been able to determine the pattern.
To answer some of the other questions...
The solenoid valves I'd estimate are 50 ft downstream of the pumps and about 15 feet higher. They are in the horizontal leg near the point where the pipeline turns up to to the roof.
Our tank is open to the atmosphere, but the top is mostly covered so debris is not much of an issue. The water in the expansion tank is in the basement of the building and at what I'd consider "normal" room temperatures.
Also, our system is only a couple years old. I believe this is only the second summer with this chillers and last year feeding the water towers was not an issue.
RE: Cooling Tower Make-up Pumps
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Cooling Tower Make-up Pumps
It seems strange to me that there could be an obstruction on the discharge line that sometimes prevents water from flowing and other not, but the solenoid valve.
RE: Cooling Tower Make-up Pumps
I have not seen this myself though. I will look on Monday to confirm.
RE: Cooling Tower Make-up Pumps
350' TDH design point
----
380' Shutoff
30 / 380 = about 8% head rise to shutoff
RE: Cooling Tower Make-up Pumps
RE: Cooling Tower Make-up Pumps
You mentioned there is no check valve in the discharge piping,and the pump is programmed to start 5 sec after the solenoid valve open.
During the 5 secs, the water in the vertical pipe will be rushing down to the suction tank and back spinning pump and motor. If the motor is energized during this time, you have been very lucky not to had a broken shaft or damaged motor.
Hearing a clicking sound at the solenoid valve does not necessary meant it is open. I would suggest you open the valve to check.
RE: Cooling Tower Make-up Pumps
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cooling Tower Make-up Pumps
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
RE: Cooling Tower Make-up Pumps
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cooling Tower Make-up Pumps
This comment is not necessarily true. If the water level is low in the cooling tower you will be drawing air into the pump suction. Put your ear to the pump piping and listen to hear that it is flowing full
RE: Cooling Tower Make-up Pumps
With the way this place moves, it'll probably be next summer before we can look at it, but I'll update this thread as I learn more.
I'll be following the thread in the meantime in case anyone else has any points to add.
Thanks for your help,
Cheers