Page 1 of 2 12 LastLast
Results 1 to 13 of 26
  1. #1
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31

    Confused Issue with CWP's

    I just started at a new building and they handed me keys and my areas of responsibility. Of course my first stop in the main plant to see what I have. I have 2 marley cooling towers, with 2 aurora 60hp 1250 gpm condensed water pumps that basically transfer water to 2 Sonex heat exchangers. I looked at the triple duty valves and noticed that they are damn near closed (at 9 on a a scale of 1-10 and 10 being closed). So I checked the suction and discharge gauges and I am reading 0 at suction and 60 at the discharge. These are 8 inch iron pipe with Victaulic couplers. I was astonished at the suction pressure. I would like to see something in the positive pressure range and I believe these to be in vacuum. The cooling tower sump level is only about 50 feet away and about 2' above the suction of the pumps and the pump is rated for 100 feet of head.

    I believe that these pumps should be on a lead/lag style operation and there is a Seimens Apogee system (haven't had access to the front end yet due to politics) and the original sequence of operations doesn't state the run status of the pumps for normal ops.

    I also found out that the triple duty valves have either been replaced or rebuilt 3 times (once on one pump and twice on the other) in the past 5 years (building commission).

    So any thoughts? I think I need to shut down one of the pumps and have it balanced again... or they have over sized the pumps for this application.

    Thanks in advance.

    Matt

  2. #2
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    5,696
    Without seeing your set up its hard to tell. At my set up the cooling towers are at or below the top of the pumps by a few feet. The pumps run 8” vacuum. When I read 14 to 16” it time to clean the strainers.

    Hope it helps

  3. #3
    Join Date
    Jul 2008
    Location
    nw ohio
    Posts
    55
    Are the pumps in series or parallel? Pumps in prallel cause problems, series boost pressure.

  4. #4
    Join Date
    Jan 2010
    Posts
    24
    Need some more info! You say you have a sump and it is 2 feet above the suction of the pump. Is the top of the water level 2 feet above or the bottom of the tank 2 feet above? You need to measure how far above the suction inlet the sump tank waterline is, then multiply that by .433. 1 foot of water equals .433 psi. So if the waterline is 10 feet above the suction inlet, the pressure is 4.3 psi. This is assuming that the sump tank is open to the atmosphere. Then if the tank is 50 foot away you need to keep in mind that the line may have "some" pressure drop. How much pressure drop is going to be determined by whether or not the line is sized correctly. Shut the pumps off and see what the pressure is, that should tell you if you have enough water (head) in the tank. "IF" this is an open tank you probably have a strainer (If not 2) in that suction line somewhere. MAKE SURE IT'S CLEAN! Then check the system.

  5. #5
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31
    they are parallel. They are open to atmosphere on the suction side. there is grating on the suction diffuser at the cooling tower. The suction line is 8" and only about 1 1/2' about the suction centerline of the pump. The strainers according to past maint. logs were checked about 3 weeks ago and had nothing in the strainers.

    Like I said, I just started last week and never saw a pump system so small with (2) 60 hp pumps rated for 1250 gpm each with 100ft head design and still have the triple duty valves at 90% closed and a 0 on the suction side. I calculated that each pump runs about 47 kwh and both pumps run 24/7. With southern california energy prices that a huge waste also. I assume I am having some cavitation due the vacuum in the suction side and I can hear it in one pump.

  6. #6
    Join Date
    Dec 2008
    Location
    Dixiana, AL
    Posts
    2,610
    What you don't say is the design GPM for the end use devices in the system (hx's, condensers, both?), if the pumps are in a manifolded (common inlet and outlet) or dedicated configuration (each pump dedicated to one device, but parallel as far as the overall system is concerned). Assuming that they are condenser water pumps (since you've labeled them as cwp's) as well as for HX's, the rated flow on your pumps - each - is good for 416 tons of cooling at 10 degrees TD on whatever it is cooling (3 GPM/ton = 10* TD). You're actually running at over 137 ft of head on the pump (60 psid x 2.317 = ft of head). That last calculation is made based on 0 psig at pump inlet because no vacuum reading was given. Regardless of that, you're still way over the rated head.

    The question is "Why?". First things first - figure out if the ratings on the pump match the system design conditions. If not, why not? If they do, then it's time to start looking at why someone has induced that much head on the pump, knowing that the final delivery volume would be short. You need the pump curves and performance data to determine if you are in a detrimental condition at the pump suction due to lack of suction head. As has been stated, some pumps will run in a vacuum on the suction - but some won't (at least not very well). It may be good to get your pump rep out to look at the situation if it's something you're not very familiar with. Since you just took over, it could be that he already knows the issues - could have been a problem since startup and the problem has been band-aided to stop the pump cavitation. Which will cause drastically increased velocities thru the triple-duty valve, causing issues with it, as well.

    A little info about yourself in your profile would go a long way, also.

  7. #7
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31
    Unfortunately the heat exchangers do not have a GPM rating and have not had a chance to contact the manufacturer. In fact I have never heard of this brand of heat exchanger. They are Sonex model 562-15230.

    The pumps are manifolded also. All the pipe connections are Victaulic and until recently Victaulic only rated their standard connection for 5 in/HG. To upgrade the connection to withstand higher vacuum a metal sleeve is intalled withing the rubber gasket. These connections are original. So not only is it cavitating due to the vacuum in the suction side, but also possibly sucking air. I do believe someone knows there are issues and just not recognizing that damn near closing the triple duty valves is not the solution. I think that this system was designed to be on a lead/lag operation and not both pumps at 24/7. Then after that have the system balanced under the load of each pump operating independently.

    I do not see a GPM designation for the Marley cooling towers. To further make my life tortured, the developer is being very difficult about getting as builts and I cannot trust any plans that I have for mechanical. For example, I have seen some plumbing plans and they state that for the sump pumps they require Myers 100 GPM x2 for each sump. While investigating why on pump is down, it is not a Myers! They are all Stancor. Even the controls are Stancor.

    This building has already had boiler design and installation issues and since commision (5 years) has already replaced (not repaired) 3 to my knowledge.

    I will update my profile if it helps. But a little history on me. I have worked in about 18 different buildings in the Orange county building as a building engineer. I have operated and maintained basic Cogen plants down to your basic carrier heat pump and all the equipment to support them. I am familiar with trane CVHE centrivac 500 ton chillers and smaller.

    Before that I was in the Marine Corps. Did 4 years as a water purification and distribution technician. At the time of my discharge I was the NCOIC in charge of the utilities platoon for MWSS-372 at Camp Pendleton and was in charge of over $30 mil in equipment and 25 marines.

    I also am very experienced in automotive A/C. Worked for over 4 years in the industry and still do free lance work for some of the A/C shops and custom builders in the Souther Ca area. Also have designed systems for Robby Gordons Baja 1000 team.

    I dont carry any technical certifications from any prestigious schools... but it doesn't make me stupid. Just like to learn as I go.

  8. #8
    Join Date
    Dec 2008
    Location
    Dixiana, AL
    Posts
    2,610
    Never meant to imply stupidity - we all like to know with whom we're speaking, especially when going in-depth on a subject such as this. There's a "Refrigeration and Icemaking" section here where you can go and read the first threads' opening post by "market tech" - explains it much better than I can.

    On the sump pumps - you probably have the "or equal" instead of the spec'ed.

    About the CWP's: Do you know what the suction pressure is? Depending on the length of run from the cooling tower, there shouldn't be too much DP on the piping, assuming correct pipe sizing. Unless someone just threw this thing together off the cuff, you should be able to go to the equipment reps and get the design specs on everything that you have. Even if it was thrown together, they can give you basic flows, DP's and DT's for the HX's and cooling towers - mfr reps won't guess, they have to be given something hard to go on or it can come back on them. Sonex is a reputable HX - our Aurora pump rep handles those, also. Your problem is fixable (maybe with less trouble than at first glance), but you're going to have to know what it's supposed to do before you can make it do what it's supposed to do. Sometimes, getting that part, is the hard part....

  9. #9
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31
    Well I guess I am trying to circumvent all the politics. I am hired under an engineering services contract. Working with the propert manager is impossible. The past contractors are less than helpful and I do have to be careful to not step on any toes as the new guy and try not to make it sound like I am challenging the designers. But, seeing that the designers gave no reason for thier design (proper plans ... etc.). Just seems like I have more work to do by contacting Sonex and Marley for GPM rates for the equipment. Hell, I may just need to call out an independent contractor to survey what we have and submit thier findings.

    I have the rest of the building on my shoulders besides the HVAC side. They have never even had an infrared inspection of any of the electrical and any time anything breaks, they just replace it no questions asked. Triple duty valve failed... so i guess we just get it replaced and ask no questions.

    Anyone in So Cal need a building engineer? I may be looking for a new job soon.

    Matt

  10. #10
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31
    I just calculated the NPSH and found it to be -4.56 but do not know the NPSHR. Wondering if I did it wrong... if anyone can help me it would be greatly appreciate it.

    thanks.

    Matt

  11. #11
    Join Date
    Jan 2010
    Posts
    24
    I'm just curious here, and please don't be offended at my questions as I am only trying to help. You are awfully concerned about how much suction pressure you have or do not have. With all due respect, you are chasing the wrong thing! You should be more concerned with TDH then plotting that on the pumps curve. That will tell you how many gpm's you are moving. Then it's up to you to determine if your GPM's are adequate. You say you're in a vacuum on the suction side, where is your guage hooked up at, is it downstream of the strainer (strainers)? Is it a compound guage, are you SURE you're in a vacuum. YOU are gonna have to pull apart all strainers and see if they are plugged. You are saying that you have -4.56 Net POSITIVE Suction Head. It's not positive suction head if it's negative! A negative would indicate you are operating under Suction LIFT conditons. That's concerning considering you stated that the sump was above the pumps. Again, how far above the suction of the pump is the waterline of the sump? That should tell you your NPSH.

  12. #12
    Join Date
    Jun 2008
    Location
    Orange County, CA
    Posts
    31
    The guage is a 0-100 and about 4' before the strainer. The strainer is located immediately prior to the impeller housing. I can guarantee I am in vacuum... how much... not sure. I was told by a contractor it was about 5"hg. But this may allow air to be sucked in at the Victaulic. I am not so much concerned at this time because if my pump impellers are damaged due to cavitation then discharge presssure and GPM will soon be 0. After I confirm and satisfied with suction pressure then I can balance gpm and discharge pressure via the throttling valves. I have plenty of room to open those.

    Does anyone agree that running 2 60hp 1250GPM pumps with 8" pipe and throttling valves near closed at about 90% is a problem and that possible running one at a time with proper balancing will solve this?

    Ultimately I am not going to pay for stuff when it breaks... but i just can't let this go and accept "thats the way it has always run".

  13. #13
    Join Date
    Jan 2010
    Posts
    24
    Does anyone agree that running 2 60hp 1250GPM pumps with 8" pipe and throttling valves near closed at about 90% is a problem and that possible running one at a time with proper balancing will solve this?

    No way I can guarantee it's a problem until it's plotted on a curve and proven. The pump curve is going to determine valve position. Suction pressure in itself means nothing. Suction pressure is only used to determine TDH. Suction pressure is only part of the equation, whether positive or negative (pressure). You're still way short on info here dude! Until you get more info you're still chasing your tail.

    Let me put it this way, You need the following to determine the answer to your problems.

    1) Accurate SP reading
    2) Accurate DP reading
    3) TDH
    4) Actual impeller size
    5) The proper pump curve
    6) Motor HP
    7) You need to be able to work between FT H2O and PSI

    Until those 7 requirements are met I can't say you have a problem with anything. Regardless of the fact you may or may not be operating in a vacuum.

    The only other thing is operating both pumps at once should not be hapening if they are truly in parallel. 2 pumps in parallel operating together are not moving more water.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event