Entire pump system cavitation

[kryanl]

We've been called in to troubleshoot a pumping system at one of our mills, which has been working fine up until now, but the operators are looking for a more efficient system. Right now, 120 F water is being pumped to a total of 4 separate users. These users range in both flow and pressure requirements. ie, one requires an estimated 500 gpm @ 45 psi, another is estimated at 3000 gpm @ 25 psi, and so on. All four control valves are throttled, obviously, a couple are nearly closed. these valves are far downstream of the pump. Our initial thought was to separate the high and low head users. But, we can't figure out quite how to size the pumps, as the above mentioned flows are just estimations. There are no installed flow meters, and when we tried using ultrasonic measurements, we found that every line is cavitating way to much to get a good reading. This is occuring everywhere, from ten feet downstream of the pump, to 100+' away, at the inlet of each control valve. Can anyone explain why the whole pumping system may be cavitating? I would understand if it was only at the outlet of teh control valve (ie, the drop across teh valve is simply too great) But, for it to be occuring at the pump? A few weeks ago, we did get an opportunity to take apart the pump, and it looks brand new on teh inside, despite the fact it is nearly 20 years old, original bronze impeller and all.

Oh, another factor that may come into play in this problem is that over the course of a few years, the mill had already removed about 1/4 of the flow that the pump had been designed for, yet never changed the trim of the impeller. (BTW, it is a double suction, originally sized for 18,000 gpm)

Thank you for any ideas / suggestions / information.

Kay Ryan

 

[PUMPDESIGNER]

The whole system cannot be cavitating.
You can have cavitation at pumps and valves, etc., but not in the delivery lines.

Cavitation is the vapor phase occurring within a fluid, due to local pressure falling below the vapor pressure of that fluid, which in your case being hot water at 120 degrees, vapor pressure would be around 3.943 Feet of head. I assume you must have more pressure than 3.943 feet by far if you are moving water down through those pipes as you report.

Find suitable locations for flow meters and pressure transducers, connect to data loggers, then draw out the system, then reverse engineer the system from scratch.

Then compare your design with what is there and make your decisions as to how best to get the job done either replacement or modify.

PUMPDESIGNER

 

[kryanl]

We did find suitable locations for flow meters. For example, our best one was a 12" line, with a straight run of pipe approximately 40' upstream, and then about 55' downstream was a 90 degree elbow. We tried using two different flow meters, each reporting errors that were indicative of cavitation. We thought at first that maybe somehow there was air in the line, but found out that that typically results in a different error code. The only pressure reading we were able to get was right at the discharge of the pump, it was 110 psig. Preliminary pump calculations indicate that only about 70 psig would normally be required. We did not try transducers. We did, however, request several hot taps be made at various points so we can install pressure gages. That will probably take a few days to complete, knowing the maintenence staff that there is. I was just trying to maybe get a little background research done. Thanks for your input.

Kay

 

[PUMPDESIGNER]

I will keep throwing in a few ideas.
Cheap way to look at pressure is a little data logger pressure recorder you can find at this link.

http://www.dicksonweb.com/product/model_343.php

You need a little software package with that unit at the following link:

http://www.dicksonweb.com/product/model_type_5.php

We have used them for years, work great.
Flow logger I have no recommendation from direct use, but Dickson also sells a cheap little data logger for flow that you can connect to a flow meter with 4-20 mA I think.

PUMPDESIGNER

 

[seymours2571]

Kryanl,

Pumpdesigner is right about the fact that your entire pump system will not be cavitating. What most likely is happening is that you could be using the wrong type of non-contact flow meter.

There are two basic approaches to non-contact flow measurement. One method requires perfectly clean fluid free of turbulent vorticies, while the other method requires it.

Take a look at the following website from Omega.com for help:

http://www.omega.com/literature/transactions/volume4/T9904-09-ELEC.html#elec_3

They have a very good tutorial on the two basic types of non-contact flow measurement.

As for your fluid delievery system problem, not sure what the answer would be. Depending on the size and cost, you could create an automated control system that monitors and controls the flow rate to each delivery branch. However, without already having some measurement devices installed, this idea could prove difficult.

Good luck,

Steve

 

[BRIS]

Pumpdesigner is already on your case and giving you good advice. Clearly your system is not cavitating - in fact you appear to have the opposite problem in that your pump is producing too much head and you have to throttle back the control valves.

Your immediate problem is that you are getting error readings from the flow meters which incorrectly indicate cavitation. - This suggests to me that you have air in the pipeline.

Have you considered installing insertion flow meters - these are small mechanical turbine meters which are installed through a pressure tapping. They are of limited accuracy but they will give you a result.

Brian