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What is TDH?

What is TDH?

What is TDH?

(OP)
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?

RE: What is TDH?

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

RE: What is TDH?

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,

RE: What is TDH?

(OP)
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?

RE: What is TDH?

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

RE: What is TDH?

(OP)
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.  

RE: What is TDH?

Roach!

Your system seems to be positive suction type. So you have to consider 14psi as discharge head.

Yes. velocity head is covered under TDH and discharge gauge will show this.

Balancing tank is a simple water holding tank.

Regards,

RE: What is TDH?

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

RE: What is TDH?

(OP)
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

RE: What is TDH?

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)

RE: What is TDH?

(OP)
SHARQ
I am not clear.  I read 28psi ( 64.5ft) of  "static head" with pump off on inlet and outlet.  With pump on,
I read 42psi(97ft) of "static pressure" at discharge. How?

Roach

RE: What is TDH?

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.

RE: What is TDH?

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.)
 

RE: What is TDH?

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

RE: What is TDH?

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.

RE: What is TDH?

BOB,PE

make it simple, and ignore my writing !

Samir

RE: What is TDH?

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.  

RE: What is TDH?

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.

RE: What is TDH?

(OP)
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

RE: What is TDH?

Where is your expansion tank located?

RE: What is TDH?

(OP)
On the return line (low pressure side) feeding into the cooling tower. The pump is on the cooling tower supply line.

RE: What is TDH?

You mention "cooling tower".  But you said it's a closed system so it's some type of closed circuit cooler I assume.

The pressure at the expansion tank should not change, whether the pump is on or off.

If the expansion tank is located too far from the pump suction, you will experience negative pressures at the pump inlet.

In most cases, you want the expansion tank as close to the pump suction as you can.  This will almost always result in positive suction pressures.

Bell and Gossett has design manuals dealing with expansion tank locations and its affect on system pressures.

RE: What is TDH?

(OP)
KLH
Yes, it is a closed circuit cooler.  The expansion tank is on the opposite side of the closed circuit cooler from the pump, but the rated pressure drop across the cooler is 3.8 psi, and there are 2 or 3 90ยบ elbows with that.  With the pump off, I read 28psi, which I assume to be the charged pressure of the expansion tank.  How is it that I create such a large vaccuum without the line losses available, unless I have something restricting flow(closed valve, dirty filter, etc.)?

RE: What is TDH?

Please explain how it is pipe, starting at the pump discharge.  It will help me understand the system.

For ex.  Pump, 30'of 4-inch pipe, closed circuit cooler, 40' of 4-inch pipe, expansion tank, 10' of 4-inch pipe, pump.  Include valves, strainers, etc and filters (you mentioned these on last post) if you have them.  

Why is the head so high? I think I saw 124' on an earlier post.

RE: What is TDH?

Please explain how it is pipe, starting at the pump discharge.  It will help me understand the system.

For ex.  Pump, 30' of 4-inch pipe, closed circuit cooler, 40' of 4-inch pipe, expansion tank, 10' of 4-inch pipe, pump.  Include valves, strainers, etc and filters (you mentioned these on last post) if you have them.  

Why is the head so high? I think I saw 124' on an earlier post.

RE: What is TDH?

(OP)
This is a very long system that I don't have time to specify, but in general, I go from the pump discharge up 30 feet out 300 ft and feed two compressors in parallel.  The return line is back 300ft and down 20 ft to a closed circuit cooler.  Most of this is 4 and 5" pipe, with various gauges, strainers, elbows, tees, ball valves,etc.  It is not worth calculating head.  I read 42psi at discharge of pump, and 24" Hg vaccuum at inlet.  This gives me a total of 124ft of head. This gives me the GPM that I was interested in finding out.  I do not meet the NPSHR, but I am very close, and I am not cavitating, so I am OK for now, but I am looking at all possible restrictions to find out why I have such a big drop in pressure.

RE: What is TDH?

Like you said before, take some pressure measurements.

I would start from the expansion tank and work my way to the pump suction.

Good luck!

RE: What is TDH?

Roach

for the problem of your water pumping system(42 psi & 28 psi case), i have an experience for the closed circuit cooling water system ( air cooler), our discharge pressure is 6 barG, static head: 28 Meters, Tdh is about 35meters(simply say, 3.5 bar), the suction pressure is 2.5 barg. an expansion vessel is just above the pump with 3 meters height which pressure set point is 2.5 barg, the total length of the pipe is about 90 meters(main pressurized pipe) , the minimum suction pressure for the system i ever seen was 1.2 barg cause the suction strainer was clogged over 60% of the area during the commissioning stage. because your system shall be positive suction head, except clogging in suction side very very seriously resut in inadequate flow for the pump, i don't think we have chance to see the vacuum suction pressure.

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