If I have a guage at the outlet of my pump on a closed H2O system, and it reads 42psi with the pump on, and when the pump is turned off it reads 28psi, does that mean that I am getting the GPM that is seen on the pump curve at 42 or 42-28=14psi ( 97 or 32 ft of head)? If it is 97, then can someone explain why my flow pressure is more than my "static" pressure. I thought that static would be more since some of the pressure energy is converted to kinetic through velocity. What exactly is my guage reading, static or "stagnation" when the pump is running?
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What is TDH?
- Thread starter Roach
- Start date
I a closed loop (pressurized) the delta P from pump outlet to pump inlet must equal the pump head because the static pressure diff. is zero. But the static pressure is not the same as the inlet pressure - inlet pressure is lower than the static. I am not 100% certain but i belive that
P_inlet = 28-(42-28)=14 psig (a or g? i assume g)
so that the "increase" when running compared to stopped from static equals "decrease" on the suction side.
Best Regards
Morten
P_inlet = 28-(42-28)=14 psig (a or g? i assume g)
so that the "increase" when running compared to stopped from static equals "decrease" on the suction side.
Best Regards
Morten
Roach!
Total Discharge Head is the sum of Static Discharge Head and Frictional losses in discharge pipeline and pressure drop across the equipment. Total discharge head required in your case is 42psi but your pump is delivering at a head of 14psi which you have to consider for flow rate measurement. (here pump is utilising the head developed by a water column in return line). If you keep a balancing tank in the same level as that of pump and discharge return line freely in it, then your pump will still deliver at a head of 42psi and you have to check flow rate at 42psi.
Regards,
Total Discharge Head is the sum of Static Discharge Head and Frictional losses in discharge pipeline and pressure drop across the equipment. Total discharge head required in your case is 42psi but your pump is delivering at a head of 14psi which you have to consider for flow rate measurement. (here pump is utilising the head developed by a water column in return line). If you keep a balancing tank in the same level as that of pump and discharge return line freely in it, then your pump will still deliver at a head of 42psi and you have to check flow rate at 42psi.
Regards,
- Thread starter
- #4
MortenA
I put a guage on the inlet, and it drops to zero immediately when the pump is turned on. I do not have a guage that will read negative (or absolute), so I don't know how far below atmospheric the pressure is droppping. Thanks for the input though.
Quark!
what about 'velocity head". Is it included in TDH? If so, is it included in the guage reading? Also, what does a balancing tank do for me? Does it serve the same purpose as an accumulator? If so, is it charged to a certain pressure? Would it look like a tall ASME coded propane tank with a flanged fitting piped to the return line?
I put a guage on the inlet, and it drops to zero immediately when the pump is turned on. I do not have a guage that will read negative (or absolute), so I don't know how far below atmospheric the pressure is droppping. Thanks for the input though.
Quark!
what about 'velocity head". Is it included in TDH? If so, is it included in the guage reading? Also, what does a balancing tank do for me? Does it serve the same purpose as an accumulator? If so, is it charged to a certain pressure? Would it look like a tall ASME coded propane tank with a flanged fitting piped to the return line?
Does the gauge stay at 0? Because if you have a large system it might take a while untill all the liquid has been accerelated and this will increase the dP at pump inlet. If its a small system it shouldnt matter.
Usually the velocity head (v^2/2g_c) can be neglected. But try to calculate it.
Best Regards
Morten
Usually the velocity head (v^2/2g_c) can be neglected. But try to calculate it.
Best Regards
Morten
- Thread starter
- #6
MortenA
I bought a vacuum gauge and measured the pump inlet pressure to be 24"Hg vacuum (-27ft). This subtracted from my discharge pressure (97ft) puts me at "TDH" of 124ft.
My pump curve tells me that I am at 170GPM (all gauge values). The system was designed for 166GPM, so I assume that this is correct. Thanks for the clairfication on this subject.
I bought a vacuum gauge and measured the pump inlet pressure to be 24"Hg vacuum (-27ft). This subtracted from my discharge pressure (97ft) puts me at "TDH" of 124ft.
My pump curve tells me that I am at 170GPM (all gauge values). The system was designed for 166GPM, so I assume that this is correct. Thanks for the clairfication on this subject.
Guest
Roach, a 27 foot vacuum on your suction is definately not a good thing!!!! Considering that water will boil around 33 feet depending on you elevation above sea level, I would assume your pump is really taking it tough. You have the pump curve, where is the operating point in reference to the BEP? way to the right I would assume!!! Let us know so that we can help some more....
Bob
Bob
- Thread starter
- #9
Bob,
At the risk of sounding ignorant, what is BEP???? NPSH on the curve is 10ft (does this mean -10ft (vacuum) or more is required, or 10ft or more is required by the system), which means I should be cavitating? I don't hear cavitation. In reference to the curve, I am dead center on the TDH and GPM axis, if my TDH is 97 + 27 ft = 124ft. Also, keep in mind that this is a closed loop system, with no vent to atmosphere. With the pump turned off, I read 28psi on both sides of the pump. My pump is the lowest elevation in the system. It doesn't make sense to me. Why such a large vacuum? Too much pressure drop in the closed loop system? Too many fittings and elbows? Please help.
Thanks
Roach
At the risk of sounding ignorant, what is BEP???? NPSH on the curve is 10ft (does this mean -10ft (vacuum) or more is required, or 10ft or more is required by the system), which means I should be cavitating? I don't hear cavitation. In reference to the curve, I am dead center on the TDH and GPM axis, if my TDH is 97 + 27 ft = 124ft. Also, keep in mind that this is a closed loop system, with no vent to atmosphere. With the pump turned off, I read 28psi on both sides of the pump. My pump is the lowest elevation in the system. It doesn't make sense to me. Why such a large vacuum? Too much pressure drop in the closed loop system? Too many fittings and elbows? Please help.
Thanks
Roach
SHARQ
Mechanical
- Apr 14, 2002
- 25
- 0
- 0
Simply, The pressure measured by the gauge located in Pump discharge is called "Static Pressure" and to distinguish it from the "Totl pressure" which includes velocity head. The velocity head acts only in the direction of flow end and, therefore, does not register on gauge which is installed with its inlet perpendicular to flow. its important that we note the diffrence between the terms "static head" and "static pressure" and apply them properly. Forexample when the pump is off there is a static head (elevation deffrence) which is 32 ft. On the other hand, when the pump is on, the static head is still 32 ft but the gauge measures a static pressure which is less than 32 ft because some has been converted to velocity head which does not register on a gauge.
I hope u have a clear idea now.
SHARQ (SAMIR)
I hope u have a clear idea now.
SHARQ (SAMIR)
- Thread starter
- #11
Roach, the TDH is calculated when the pump is running.
So, if you are measuring the suction pressure as -11.8 psig (24" vacuum) and the discharge pressure as 42 psig (both need to be taken with the pump running), the differential pressure is 53.8 psi or 124 feet. To be able to say whether the pump is working as per design, better or worse, you'll need to be able to measure or estimate the water flow through the system. Comparing the design point of the pump to the current operating point doesn't tell you the pump's condition.
Roach, BEP stands for Best Efficiency Point. If you look on a pump curve, the BEP corresponds, with a particular header, the point on the pump curve with the highest pump efficiency.
So, if you are measuring the suction pressure as -11.8 psig (24" vacuum) and the discharge pressure as 42 psig (both need to be taken with the pump running), the differential pressure is 53.8 psi or 124 feet. To be able to say whether the pump is working as per design, better or worse, you'll need to be able to measure or estimate the water flow through the system. Comparing the design point of the pump to the current operating point doesn't tell you the pump's condition.
Roach, BEP stands for Best Efficiency Point. If you look on a pump curve, the BEP corresponds, with a particular header, the point on the pump curve with the highest pump efficiency.
SHARQ
Mechanical
- Apr 14, 2002
- 25
- 0
- 0
I was only describing to u what is the static pressure means. Now;
When pump is off, the ststic pressure =static head =64.5ft
which is the gauge indicator reading.
when the pump is on, the gauge pressure reads only the discharge pressure = 97ft. !
Discharge head = TDH + Suction head
suction head = static head + Vel. head - friction head across suction (bernouli Equ.)
When pump is off, the ststic pressure =static head =64.5ft
which is the gauge indicator reading.
when the pump is on, the gauge pressure reads only the discharge pressure = 97ft. !
Discharge head = TDH + Suction head
suction head = static head + Vel. head - friction head across suction (bernouli Equ.)
Roach
I think you got it right. I also considered mentioning that your pressure at the suction side was near water boiling point. But i was more curious why this "inbalance" exists. I dont really understand why you dont get the same drop on the suction side as you get increase on the discharge side when compared to the static pressure. Could anybody enlight me?
Best Regards
Morten
I think you got it right. I also considered mentioning that your pressure at the suction side was near water boiling point. But i was more curious why this "inbalance" exists. I dont really understand why you dont get the same drop on the suction side as you get increase on the discharge side when compared to the static pressure. Could anybody enlight me?
Best Regards
Morten
Guest
ROACH
There does appear to be a lot of pressure drop in your system, which is ok. The manufacturer is calling for a NPSH of positive 10 feet on the pump, and you have a negative 27 foot available NPSH. The pump in my mind appears to be too small for the system you have. Taking into account the flow you need and the head losses, you should pick a pump that gives you a minimum of positive 10 feet on the suction side when operating.
SHARQ
Buddy, go read your books again before you confuse people.
There does appear to be a lot of pressure drop in your system, which is ok. The manufacturer is calling for a NPSH of positive 10 feet on the pump, and you have a negative 27 foot available NPSH. The pump in my mind appears to be too small for the system you have. Taking into account the flow you need and the head losses, you should pick a pump that gives you a minimum of positive 10 feet on the suction side when operating.
SHARQ
Buddy, go read your books again before you confuse people.
Chillermec
Mechanical
Not that I am a pump application whiz, but, I have never seen a closed loop pump operating in suction vacuum without something being wrong. My first suspect is a suction strainer is prior to the suction gauge that is plugged substantially. After that, is the highest point in the piping system above 64 feet? If so you have an underfilled system. Lastly, something in the discharge line may be closed off. ( butterfly or control valves ) I'd be curious for you to check these items.
stressriser
Mechanical
I think right now, the previous two posts are correct, there is something wrong with the inlet supply. Vapor pressure of water at 68F is only 48.9#/ft^2. (less than a foot)
27-feet of vac indicates a sever flow supply restriction. I would look at the supply line first, and then move on once you get rid of all that vacuum. The only time our pumps pull that much vac is when filters are clogged or a valve is closed.
27-feet of vac indicates a sever flow supply restriction. I would look at the supply line first, and then move on once you get rid of all that vacuum. The only time our pumps pull that much vac is when filters are clogged or a valve is closed.
- Thread starter
- #19
Thanks everyone for the input.
After a lot of searching, I have found Cameron's Hydraulic Data book, which explains that NPSHR on the pump curve is measured in feet absolute. The curve requires 10 feet (abs.), and I am around 8 after taking into consideration guage heights, vapor pressure of 3%E.G. solution, and velocity head. I am sure the curve has a certain amount of cushion on the #s, and that is why I am not cavitating. I don't know how much pressure loss I am seeing through each part of the system, since the installation did not include guages at the critical locations, so I don't know if there are any restrictions, or if they are there for a reason (i.e. for required pressure drops for equipment tied to system). I am trying to set up guages at various locations, which will tell me where I am loosing that much pressure on intake. Probably a filter if I know our PM habits. Again thanks for the help. I have learned a lot from the discussions.
Roach
After a lot of searching, I have found Cameron's Hydraulic Data book, which explains that NPSHR on the pump curve is measured in feet absolute. The curve requires 10 feet (abs.), and I am around 8 after taking into consideration guage heights, vapor pressure of 3%E.G. solution, and velocity head. I am sure the curve has a certain amount of cushion on the #s, and that is why I am not cavitating. I don't know how much pressure loss I am seeing through each part of the system, since the installation did not include guages at the critical locations, so I don't know if there are any restrictions, or if they are there for a reason (i.e. for required pressure drops for equipment tied to system). I am trying to set up guages at various locations, which will tell me where I am loosing that much pressure on intake. Probably a filter if I know our PM habits. Again thanks for the help. I have learned a lot from the discussions.
Roach
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