PumpDude
Mechanical
- Feb 5, 2004
- 8
I have 6 single stage split case pumps in a chiller system. Half on the evaporator side and half on the condenser side. All pumps are identical in construction and configuration, i.e. impeller diameter, speed, motor hp, etc. One set pulls water directly from a cooling tower through what has been assessed as a excellent design according to HI standards. (more than 10 pipe diameters from the pump suction to the elbow/side outlet T, eccentric reducer with flat side up, etc.) The other set of 3 has the same plumbing configuration on the suction side and only has two differences from the other. On the discharge plumbing an orifice plate has been added to add head and the water supply is from the discharge plumbing of another system. I am experiencing the same problem with these NEW pumps. All exhibit cavitation and after pulling the top of one of the pumps I am finding circular pitting (appears to be hit with a ball peen hammer....and I am being told by the mfg these pumps are not cavitating!!!!!!!)on the volute on both sides of the split leading up the the eyes of the impeller on the suction side. The discharge side as well as the impeller itself shows no damage. There also appears to be a small amount of "blow by" between the case/volute and casing wear rings (lending to the theory of internal recirculation). I have only pulled one top but all pumps had the same sound signature and vibrations plot. We choked the discharge from wide open to nearly completely closed with little to no change in the sound or vibrations. The pump we inspected the internals on was sent to the mfg. and tested in their lab to no avail. We provided all of the current operating parameters and it appears they were close on most, except the plumbing on the suction side. They have stated they heard none of the noise and supplied vibrations data showing nothing like we found in the field. The differences in the suction plumbing was that we have an isolation (butterfly) valve, then a check valve, then an expansion joint, the reducer up to the suction flange of the pump. They had what appeared to be an approx. 6 foot run of pipe from an elbow into the suction of the pump. Prior to the elbow was their isolation valve. Having not repeated our symptoms in the lab they began offering opinions on why this is happening in the field. We have discussed and dismissed NPSH issues and suction plumbing design faults. We have also discussed disolved oyxgen, entrained air, internal recirculation and flow vortices. We are currently waiting on an analysis on the fluid and I will post those results. I appologize for the length of this post, but I wanted to convey all that we have done and found so far. I am hoping that some one out there has had this happen and can tell me what was done to alleviate it. We are getting nothing from the pump mfg. and the rest of us (rep from the distributor, pump engineers from my home office, myself and personnel here on site) continue to "scratch our heads" in frustration. Thanks in advance for taking the time to provide me with whatever insight you can.
