At Open valve conditions it should show pressure of zero ft or near Zero,Thats the only condition you can achieve max discharge.

It seams to me that, when you say 2" ball valve its not the size of max opening of valve.may be its its fitting sizes.you need make sure that the size of valve opening is more then the inner diameter of pipes itself.

Sorry I forget to add that you need to close valve totally to get max head value.

You can plot a complete head/flow graph by opening valve slowly from total close to total open,

Yes you are measuring "true head" and these flows/heads should match the pumping curve.

500 L/Min is around 8.33 L/S and with this flow rate and 2" pipe your pressure drop is around 2500 Pa/m which means you are running the flow at about 3.7 to 3.8 m/s which is very high with high pipe friction.

When running 100L/Min or 1.67 L/S you are in range of 140 Pa/m pressure drop and less than 1 m/s flow velocity.

So you are running now at very low pressure side of flow run.

Therefore just see the pump curve or if you can not find it, then use first university course book in fluid mechanics and sure you will get your answer.

You need to measure the "net total head" of the pump in order to compare against the published performance curve. This requires that you also measure the head at the inlet of the pump. Since both the inlet and outlet are 2" diameter the velocity heads will be equal and can be ignored in the calculation. Therefore, the difference between the head measured at the 2" outlet and the head measured at the 2" inlet will be the "net total head" and should closely match the performance curve of the pump.

The head measured with the valve full open is the system resistance of the discharge pipe plus any static head in the discharge line. This resistance will depend on the size and length of the discharge line. Note that the pressure guage calibrated in ft is only correct for a certain specific gravity of liquid, normally water at SG=1. If you wish to use a more normal pressure guage which reads in psi, the conversion for water is 1 psi =2.31 ft of water.

If there is a negative pressure at the suction side add it to to the positive side pressue. ie -10 ft Suction and +60 psi disch is a total of +70 ft gauge head. Add to this velocity head at the suction and discharge sides so you have 10+vhs+60+vhd and then you have the head in feet of head for the pump job. I assume you are pumping water.

From the information that you have given, it would appear that there is either a considerable amount of discharge piping before the ball valve or a considerable static head between the ball valve and the pump. The gage you have that reads in feet is a very good way to measure the discharge head of a centrifugal pump. It eliminates the need to calculate the head from a pressure reading. And, contrary to the information given in another reply, this head being read in feet is equally as accurate as a gage reading in PSI. Both measures are only as accurate as the type of gage you are employing. If the gage reads 110 feet it could just as easily read 47.66 psi simply by installing a suitably calibrated dial face on the gage. The mechanics of the gage would remain the same and the reading accuracy would be the same. No difference.

Now, to try to enlighten you about your question. It would be very helpful if you could post a sketch showing the installation with the locations of the pumps, the gage, the discharge and the suction line. Also, is there any pressure on the inlet? A centrifugal pump will add its pressure to any inlet pressure. So the reading of 60 feet of head with the valve wide open is the head that is being developed at the point where the gage is positioned. If there is a positive suction head, that will be added to the pressure the pump is developing and will be read as "Total Dynamic Head" at the gage.

And, as per your question, that is definitely the "true head." That is actual reading that will be as "true" as the ability of the gage used to measure it. You can, with that gage, plot a system head curve for the piping system through which you are pumping simply by measuring the flow and head pressure at various points and then plot them on an X-Y axis with X being the head pressure and Y being the flow. Start with the discharge valve fully closed. Measure the pressure there and make a point on the graph. That will be zero discharge and maximum pressure (shut-off head). Then open the valve a little and measure the flow and the head and mark the graph accordingly. Afte you have done this, you will have defveloped a locus points that will define the characteristics of the pump. At least as it pertains to the flow and head.

There is a lot of information that can be gained from this exercise. You can predict the actions of the pump by using this developed curve. If you have the instruments to measure the electrical characteristics of the motor at these various points and plot them as well, you can determine the hydraulic efficiency of the pump and the BHP that will be exerted. But, alas, more information is required as stated above. Interesting question.

And I forgot to sign in. The post #7 above is me. Sorry folks.