Problems with orifice plates

Principle of orifice plate flow meter: An orifice plate is installed in the pipeline, with pressure measurement tubes connected to both sides of the orifice plate, which are in turn connected to a U-tube manometer. A orifice flow meter utilizes the throttling effect of fluid passing through a sharp orifice, which increases the flow velocity and decreases the pressure, thereby creating a pressure difference before and after the orifice that is used as the basis for measurement. If the diameter of the pipeline is d1, the diameter of the sharp orifice in the orifice plate is d0, the diameter of the constriction formed as the fluid passes through the orifice plate is d2, and the density of the fluid is ρ. At points I and II on the interface, namely at the pressure measurement probe in front of the orifice plate and at the throat, the velocities are u1 and u2 respectively, and the pressures are p1 and p2. According to Bernoulli’s equation, ignoring energy losses, we have: (u2² – u1²)/2 = (p1 – p2)/ρ = gh, or (u2² – u1²)¹/² = (2gh)¹/². Since the position of the throat changes with the flow velocity, and it is difficult to determine the cross-sectional area S2, whereas the area of the orifice is known, and the position of the pressure measurement point remains unchanged after the device is manufactured, therefore u2 is replaced by u0, which corresponds to the velocity at the orifice diameter. Additionally, energy losses due to local resistance in the fluid are taken into account, and a correction factor C is used for adjustment. Then we have: (u02 – u12)1/2 = C(2gh)1/2. For incompressible fluids, according to the continuity equation, we also have: u1 = u0S0/S1. After some rearrangements, it can be shown that u0 = C*(2gh)1/2 / 1/2. If we define C0 as C/1/2, then the expression can be simplified further to: u0 = C0(2gh)1/2. Using u0 and S2, it is possible to calculate the volumetric flow rate of the fluid: Vs = u0 * S0 = C0 * S0 * (2gh)1/2, or Vs = C0 * S0 * 1/2. Here, R represents the reading of the U-tube manometer ； ρr—is the density of the liquid in the manometer, ； C0—is the orifice flow coefficient, which is determined by the shape of the sharp edge of the orifice plate, the location of the pressure tap, the ratio of the orifice diameter to the pipe diameter, and the Reynolds number; its specific value is determined through experiments. When d1/d2 is constant, once the Re value exceeds a certain threshold, C0 approaches a constant value. Generally, in industry, orifice plate flowmeters are specified to be used under flow conditions with a constant C0. The orifice plate measures volumetric flow rate. If the material of the orifice plate is suitable, the medium does not corrode the orifice plate, and its density is known, it can be used to measure other media. No need to take the square root.

Why are mass units provided in the orifice plate calculation specifications? For example, T/h, Kg/s?

It should be the mass flow rate obtained by converting the volumetric flow rate measured by the orifice plate under the design conditions.

Where do I convert it to mass flow rate? In the transmitter or in the DCS? Do all orifice plates measure volumetric flow rate? In the orifice plate specification calculation sheet, the flow rates are sometimes given in volume terms and sometimes in mass terms – why is that?

1 The orifice plate is a differential pressure flow meter; the volumetric flow rate and mass flow rate measured can be converted into one another. 2. The orifice plate can measure different media as long as its applicable range is maintained, but the differential pressure must be recalculated. The differential pressure measured by a 3-port orifice plate needs to be squared in order to be correlated with flow rate; the range of mass flow rate that can be obtained through calculations for a 4-port orifice plate is set within the DCS. In the calculations for the 5-hole plate, some values are given in terms of volume while others are in terms of mass; I understand this to be a consistency issue, and you can unify these values after performing the calculations.

Thanks for the reply from above! So, does the orifice plate still measure volumetric flow rate? In my understanding, when a orifice plate is manufactured, it is done in accordance with the process conditions specified in the design specifications; if the flow rate unit specified is volume, then the orifice plate will measure volumetric flow rate, and vice versa for mass flow rate. Moreover, after the orifice plate is processed, if the process conditions change, the measured results will be inaccurate; it’s not just a matter of recalculating the pressure difference.

An orifice plate definitely measures volumetric flow rate. As for mass flow rate, it is a choice that depends on the process conditions; generally, process engineers prefer to specify mass flow rate or volumetric flow rate under standard conditions. In fact, if you specify volumetric flow rate along with density values, you can easily convert them yourself. After the orifice plate is manufactured, if the process conditions change, it is usually necessary to recalculate the differential pressure and re-calibrate it.

Is it necessary to change the orifice plate for recalibration? It should be volumetric flow rate. Volumetric flow rate is proportional to the square of the pressure difference. The volumetric flow rate of the fluid can be calculated using u0 and S2: Vs = u0 * S0 = C0 * S0 * (2gh)1/2, or Vs = C0 * S0 * 1/2

There is no need to change the orifice plate; calibration can be done in the DCS.

When the process conditions change, the newly calculated differential pressure is reset in the transmitter, and the DCS is adjusted to the corresponding range accordingly. If the conditions do not change significantly, there is no need to modify the orifice plate.

The square root function is implemented within the transmitter; if the transmitter lacks this function, the DCS can still perform the square root operation. If the process conditions change little, adjusting the transmitter’s range is sufficient; if the changes are significant, it is necessary to enlarge the aperture or replace the orifice plate

Got it. Thank you to the comrades above for their prompt answers.

I happen to be able to go on a quest! Thank you! :time: