Is there negative pressure after the orifice flow meter, and does it affect the measurement results?
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During testing, due to the low head of the pump, the pressure behind the orifice plate is negative at high flow rates, while the pressures before and after the orifice plate are positive at low flow rates. I would like to ask whether a negative pressure behind the orifice plate affects the flow measurement results. Too large or too small.
The throttling mechanism of an orifice plate is related to flow velocity and not to pressure. A higher flow velocity results in a greater pressure difference acting on the orifice plate, while a lower flow velocity leads to a smaller pressure difference. It is sufficient to ensure that there is a proper pressure difference across the orifice plate at low flow rates; the orifice plate has a minimum flow limit. Inform the manufacturer of the minimum flow rate, normal flow rate, and maximum flow rate, and ask them to prepare a calculation document for the flow rates. Measure the liquid to ensure that the flow meter is filled with liquid; the installation requirements need not be repeated here
Your experimental setup is not standard. The experimental results are definitely meaningless. Inspect the downstream outlet of the testing apparatus; the downstream piping should discharge at a higher level than the experimental section, and there should be no downward-flowing pipes.
It’s no problem to use an orifice plate as a standard throttling device for measurement; the issue is that your description isn’t very clear – how do you obtain the negative pressure, and has a differential pressure transmitter been installed?
We are the pump testing pipeline: the axial flow pump draws water from the tank, passes through a straight pipe section of a specified length to reach the orifice plate; there is another straight pipe section of a specified length behind the orifice plate, followed by a valve, and after the valve there is an elbow that leads back to the tank for circulation. Ordinary valves ensure that the pipeline ahead remains at positive pressure with full flow. In front of and behind the orifice plate, pressure taps are used to connect to both ends of the differential pressure transmitter; each of these pressure taps is equipped with a vent valve. During testing, we vent the pressure taps located in front of and behind the orifice plate to ensure measurement accuracy. Under normal circumstances, water can be discharged; when all the air is removed and water continues to flow out, it indicates that there is positive pressure on both the upstream and downstream sides. However, this axial flow pump has a very low head – at its rated flow rate, the head is only around 10 meters, and it’s even lower at higher flow rates. At high flow rates, the head loss due to the orifice plate increases, while the head that the pump can provide decreases as well. As a result, negative pressure exists behind the orifice plate. In such cases, when the exhaust valve behind the orifice plate is opened, no water flows out; instead, air flows in. So, determine the negative pressure that follows. As mentioned on the third floor earlier, the setup is not proper; the downstream pipes should be located at a higher level than the experimental section, and there should be no pipes running in a downward direction. This was tried by installing two 90-degree elbows with their ends facing downward into the water, but it still didn’t work. The main reason for this is the low head pressure of the pump, coupled with the throttling effect of the orifice plate, which becomes more significant at high flow rates. I know that electromagnetic flowmeters require the pipelines before and after them to be at full flow, that is, under positive pressure, but I’m not sure if orifice plate flowmeters have the same requirement; that’s why I’d like to ask about this. If anyone knows anything about this, I hope they can help.
The orifice plates are the same; all flow meters must be suitable for full-flow conditions in closed pipelines. In your case, there is a mixture of gas and liquid present, and the flow is not at full pipe capacity
The range of the orifice plate is inappropriate (too small); it is necessary to recheck the flow parameters and calculate the β value for the orifice plate.
You said it’s a pump testing pipeline, and the pump’s head is particularly low; so could it be that the testing device isn’t compatible with the pump?
There’s nothing that can be done about this – the pump has a low head, and it’s not possible to increase the pressure by using a pre-pump in an open-loop system. As for replacing it with an electromagnetic pump, the 1200 model, frankly, isn’t very reliable in terms of performance; it’s hard to say what the actual situation will be
My opinion is as follows: 1. For liquid measurement, as long as there is no flashing (formation of a gas phase) inside the pressure tap and orifice plate, it is still possible to carry out measurements. 2. “The vent valve behind the orifice plate does not allow water to flow out; instead, it allows air to flow in. It does not mean that the liquid phase at the pressure measurement point is flashing, as the saturated vapor pressure of water at room temperature is less than 10 KPaA; only when the pressure is below this value will the liquid flash. The original poster can determine this by checking the pressure gauge. 3. To determine whether flashing occurs inside the orifice plate, you can use the calculation formula for choked flow in control valves; a pressure recovery coefficient of 0.55 is applicable to butterfly valves, and this value can be used to calculate the allowable pressure drop under choked flow conditions. For the checks mentioned above, it is necessary to know the pressures P1/P2 before and after the orifice plate; the pressure measurement point for P2 should be located 5D away from the orifice plate. In this way, the permanent pressure drop across the orifice plate, namely P1-P2 (not the differential pressure value read from the orifice plate), can be determined. This value is then compared with the allowable pressure drop specified in point 3, in order to roughly determine whether flashover is occurring inside the orifice plate. This is merely a theoretical assessment; after all, the P1/P2 readings are more accurate, so a transmitter might be more suitable. Finally, I conducted a simulation using 0.1 BarG at the upstream side, a differential pressure of 0.3 Bar, and water at 40 degrees Celsius. Flashing can occur only when the differential pressure P1-P2 exceeds 0.3 Bar; however, the permanent pressure drop across the orifice plate is not that large, so I believe flashing will not take place inside the orifice plate. The liquid gravity flow enters the vacuum tower; orifice plates are also used in this way to monitor flashing.