quick question on pressure

jacobsond · May 2020

Commercial building. Activity center. Steam heat exchanger converting to a hot water system. The building is 3 stories. Should the suction pressure on the pump be 15lbs or higher? Having trouble with air in the system.Loosing pressure and pump cavitation. Not many of us are real familiar with the building. I found some auto bleeders yesterday up in the ceiling on the 3rd floor. In places with poor axcess of course. Tag on the suction gauge indicates 15lbs. Other guys never set it that high because the outlet pressure is then 40+. With a relief at 50 that's kind of close. Heating plant only monitors the output. Just plain water in the system because of possible leaks that we cannot find. Another project for the new guy I guess.

Jamie Hall · May 2020

Somethings wrong here.

The static pressure in the water side of the system probably should be around 25 pounds measured at the lowest point -- in the basement mechanical room, perhaps.

The pump, or pumps, should generate just enough pressure difference on that to circulate the water; unless you are running a lot of water through tiny pipes or partly closed valves, I'd be really surprised if that took more than 10 psi pressure difference across the pump. If it takes more pressure than that to get the circulation you need, you may have to repipe things so that you have more individual parallel circuits.

That said, then consider the pump: the inlet pressure should certainly not be less than atmospheric (vacuum). If that 15 psi you quote is real, however, you shouldn't be having any problems with cavitation or sucking air -- at the pump. But 15 psi at the pump when it is running, unless that is the lowest pressure point in the system, is likely to translate to even lower pressures elsewhere in the system, and then you can suck in air. Further, if that 15 psi at the pump inlet is real, then there shouldn't be a cavitation problem.

So... bottom line. Some clarification here, please?

Zman · May 2020

Is the circulator "pumping away" from the expansion tank? What does the gauge read with the circulator off? 15 PSI static might be a little low for a 3 story building. 30' / 2.31 = 13 psi + 5 psi (safety) = 17 psi.

What gets my attention is the 25psi being generated by the circulator. Unless they ran super long loops, that is way too much pump. Tell us a little more about the system.

GroundUp · May 2020

What they said. I'd cool it down and verify static is proper with 5psi at the highest point for safety, then isolate the expansion tank and drain it to verify air pressure in the bladder (if it uses a bladder at the pump(s) and not a compression tank up top). Sounds like your discharge piping is too small or the pump too big- is it on a VFD with a DP switch?

jacobsond · May 2020

Ill try to come up with the answers. I'm in the process of figuring out the system now. Static is at 15 now. Running the static runs about the same on the input. Output jumps to about 40 after the pump settles down. Its a fairly large Taco pump. There is 2 of them. We switch between. 3/4 pipe on the radiators. Not sure how big the supply going up is. Not likely more than 1. The high output pressure is my concern. Expansion does not have a bladder. It probally holds more than 25 gal and was 3/4 full. I have dropped it down by adding air and draining it.

GroundUp · May 2020

If the discharge piping is only 1", it's too restricted and that's where your high discharge pressure is coming from. Are these regular residential style circ pumps or centrifugal? Some pictures would be great when you get back over there, as well ad model numbers from the pumps and whether or not there is some sort of DP switch or VF drives for the pumps.

hot_rod · May 2020

Any idea the load of the building? That would shed some light on pipe sizing

Jamie Hall · May 2020

I might add that unless you are measuring the pump inlet pressure right at the pump inlet, unless your inlet piping is generously sized -- and straight -- your cavitation problems may be coming from either turbulence or pressure drop in the inlet piping, even wit 15 psi static.

EBEBRATT-Ed · May 2020

@jacobsond

The first thing we need to know as mentioned by @Zman above is are the circulators pumping toward the expansion tank (hopefully not) or away from the expansion tank (Much better)

We can't determine anything about pressure without that. You should shut the system down and put a gauge on the highest point in the system and check the static pressure with the pump off. 4-5 psi at the high point is fine. Or measure difference in height from the heat exchanger or expansion tank to the highest point in the system and allow 1 psi for every 2' of height
Drop a rope out the third story window let it hit the ground tie a knot in it pull it back up to measure the height

But, here's one big problem you CAN"T us automatic air vents with a compression tank. That where your air is coming from. The auto vents will let air into the system.

You can only use auto vents with a bladder style EX tank.

Auto vents will lead to the compression tank water logging

jacobsond · May 2020

No auto vents you say. Thats interesting. My concern also. Espically when the static falls below 10. The one in the mech room is has a valve normally in the closed position. The ones I found in the ceiling on 3 rd floor have been left open. No one knew about those.This system slowly looses pressure. The vents on 3rd look like they have been weeping. Maybe I could just shut off the valves and only open if there is an air issue. Just to be sure everyone knows the size of building. Activity center. 1 main basketball court. 1 secondary on 2nd floor. Full size pool. Plus other various workout areas. Main heat is with 7 AHUs steam coils. The hot water radiant is for halls,offices and classrooms. The lines going out of the mech room are 4 inch. The wall heaters are 3/4. Up in the ceiling on 3rd floor Not sure if the pipe is 1in or 3/4 I will have to pull insulation to be sure.I will discuss with the crew Lead about isolating the auto bleeders. After I got the air out and static pressure of 15 seems to be running good. I am going to monitor closer the rest of the heating season. Which I hope ends this week.

Ironman · May 2020

For clarity: elevation has nothing to do with pump sizing and performance in a CLOSED loop. The pump only has to produce enough pressure differential to overcome the friction of the piping (as Jamie alluded), not to overcome the elevation. Doing that is the job of the static fill pressure ( 1psi for every 2.31ft. + 5 psi for a safety margin).

The info about the pump model and whether it's pumping towards or away from the compression tank connection is crucial in determining the cause of cavitation. Please provided that so we can help find your problem.

quick question on pressure

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