Selecting the Right Pressure Gauge for a Motorized Pump

where is the most important information ? the pressure at outlet ?

if course it can be calculated, and it almost comes to be 0.9 bar (assuming water)

Then your pressure gauge may be accordingly chosen.

But then why to guess ? find out the pump spec and the outlet pressure and accordingly chose the gauge.

sb -- GA from me. I think 60% efficiency is about what you were figuring on and suggests a shutoff head maybe 1.5 to 2 times the 0.9 bar given pressure; so the gauge ought to have a range of around zero to 4-5bar. I'd try to locate it a meter or two downstream from the discharge flange as long as there isn't another fitting in the way.

I'm too sleepy tonight to do the metric velocity head correction and get it right. I'm guessing it's a 4" (or the metric equivalent) discharge line. I think this guy will need that number plus plus some explanation of what it's all about.

Ed

• What is the working fluid?
• What is its temperature?
• What is the design pressure of the pipeline?
• What are the materials of construction of the pipeline?
• What is the diameter of the pipeline?
• What is the pump outlet pressure?
• How far away does the pressure gauge need to be read?
• Is the gauge to be self-standing, or is it to be mounted on some sort of backplate?

by right pressure guage, if you mean the type, generally used guage is the bourdon tube type.

if you want to know the rating of the guage, it is recommended to use twice the shut off head(the pressure indicated when the delivery is completely closed which is also the maximum head the pump can develop) in the case of a centrifugal pump.

if the pump has a nameplate, it will carry details of head and discharge at best efficiency point. to be on the safer side, use a guage three times this value.

motor kilowattage has nothing to do with the pressure developed by a pump. it is a function of perepheral speed of the impeller which in turn depends upon the pump speed and diameter of the impeller.

the formula for the maximum head developed by a pump is

H = (Dn/1840Kn)^2 where H is head in feet, D is impeller dia in inches, n is rpm and kn is pump design coefficient. Kn may vary between .95 and 1.03 for normal impeller designs. generally using 1.03 is accurate enough. you can measure the impeller dia; as for the motor speed, nominal speed for a 50 hertz power supply is 2900 for a two pole motor and 1450 for a four pole motor. refer to motor nameplate for speed or measure using a tachometer

upsidedown -- You need to read between the lines of the original question. This fellow seems to know little about pumps. He is likely trying to deal with a problem by seeking out advice from someone more knowledgeable, perhaps even the pump manufacturer. The first question any of us would ask him in that situation is what pressure the pump is developing in the system when operating at a known flow.

To select a gauge we really need some approximation of the pressure the pump can develop. Yes, it would be nice to run the pump at shutoff; but that may not be an option. And that would not tell him anything without a pressure gauge already installed or a convenient way of measuring electric current. If a pump performance curve is available then the answer is at hand. A pump that size will likely have an impeller of diameter matched to the original application for which it was sold. Common practice is for pump manufacturers to mark that size directly on the pump nameplate. Perhaps the nameplate is absent. Perhaps the manufacturer is unknown.

So he is off to find the right pressure gauge that will give him a reasonably accurate reading and then figure out how to install it. Our answers so far have been a bit weak on the installation part.

But we can reasonably figure the pressure range with a bit of back calculating. Which is what we (ky and myself) did. We know the pump has a 10hp motor. Induction motors (a reasonable assumption here)usually have a 1.0 to 1.15 service factor in that range and are commonly matched to the operating conditions the pump was originally sold for. Pumps that size are normally selected to match maximum efficiency with the conditions of service (pressure and flow rate). So it's reasonable to assume that original selection of pump COS (conditions of service) and motor would use in the range of 7.5 to 10hp.

Since HP = (gpm x psi)/(efficiency x 1715) we will need to assume an efficiency and a power to calculate pressure. A first iteration with 0.60 efficiency(a good conservative approximation for most centrifugal pump applications that have any engineering at all behind them) and 10hp gives approximately 15psi. This suggests a rather high specific speed pump and a power curve that rises with flow rate even as the pressure produced goes down. Note the 1715 constant is only good for the English units I use. Once you have your pressure gauge in place you can test for shutoff head by closing a valve somewhere in the discharge line. And if you have a way to measure the flow rate (such as how long it takes to raise the level in a tank of known dimensions or drop the level in the suction tank) at different valve settings then you are ready to develop an actual performance curve. Power at each setting can be measured with a volt meter and a clamp-on ammeter if your electrician has a fair estimate of the local power factor.

OK, about the pressure gauge. Common bourdon tube pressure gauges in the English system are generally made in ranges or 0-15psi, 0-30psi, 0-60psi, 0-100psi,0-160psi and up. O-15 psi would give the best readings; but might get a bit out of it's calibrated range, especially if there is some suction side positive pressure like from an elevated tank. 0-30psi is probably the best choice although 0-60psi would likely give reasonable readings. Convert these readings to metric as appropriate.

You are going to need a small threaded hole somewhere on the discharge side close to the pump in a length of straight pipe 10 pipe diameters away from any fittings. Drill and thread a hole for a pipe fitting of small size. You will need to install a short pipe nipple, a small needle or globe valve and the pressure gauge on top of the valve. Setting the valve just barely open will dampen the wiggling of the needle. Closing the valve will allow you to remove and or easily change to a better pressure gauge. Note here that if you can obtain a vehicle tire valve with the right threads (you could solder a metal valve stem to a small pipe nipple) you can use a good tire pressure gauge to take your readings if it can read in the 10-15 psi range.

Now you have a way to measure pressure loss in the pipeline at whatever is the flow rate. Remember that the pressure that the pump adds to the liquid is the difference between the suction pressure and the discharge pressure. So you need a way to measure suction pressure. If the suction piping between the tank and the pump is short and maybe 6 inches (150mm) or larger the pipe friction loss will be small. The suction pressure is just the height of the liquid level in the tank measured to the centerline of the pump suction opening times the specific gravity of the liquid. Remember it's a minus number if the tank is below the pump. A better way to do this as long as you are going to go to the trouble of putting in one pressure tap is to put another one (with a valve) on the suction side, preferably at the same height as the tap on the discharge side so you don't have to add a gauge height correction in your readings. If the suction side is a lift from a lower tank or an excessively small pipe from an elevated tank you will need a vacuum gauge. An ordinary automotive vacuum gauge should be accurate enough for this job.

And while we're on the subject, a velocity head correction will need to be made for best accuracy in all calculations where the pipe size is different between the suction and discharge of the pump.

Upsidedown's formula for the impeller will be useful once you know the impeller diameter. Taking the pump apart to find that out is probably not real high on the pump owner/operator's list of good ideas. But note, many manufacturers of pumps this size will keep records of impeller diameters since lathe trimming of impeller diameter is common practice to insure that the pump operates at the optimum efficiency. This is more likely if the pump has a distinct serial number or order number on the nameplate. Sometimes they will even stamp the impeller diameter on the pump nameplate, as I mentioned above. This is pretty critical information if they ever have to furnish a spare or repair impeller.
Hope this is all helpful.

Ed Weldon

Hi,

You want a simple answer no doubt! You know the application and what the static head is so the pump will be adding something to that. You will need the manufacturers pump curve to see what the maximum head that the pump can develop. Otherwise this comes down to trial and error. Get a test gauge and perform the above test, then select a gauge which works at 50% Full Scale Deflection when the pump is delivering the required output.

Quality comes in when you have to decide whether there are a lot of dynamic pressure fluctuations present as these can destroy a cheap gauge very quickly; snubbers or air breaks can over come these problems.