Flow Orifice Turn Down Ratio

Orifice, combined with differential pressure transmitter, is the mostly used flow measurement device in oil & gas plant because of low cost and its ease of installation and maintenance. However, orifice only allows 3:1 rangeability to maintain accuracy. This means, if one wants to measure maximum flow of 10 MMscfd, then the minimum flow measurement that could be measured is only 3.3 MMscfd.

According to Bernoulli principle, the relationship between flow and pressure drop of fluid passing through an orifice is given by the following formulae:

Q = Volumetric flowrate
dP = Measured pressure drop ρ = Density
Ao = Cross-sectional orifice area
Cf = Constant

Hence, flow value could be obtained by measuring pressure drop and the relationship between them is square root.
Let’s say the orifice is intended for flow measurement of 0-10 MMscfd which is represented by 0-100 inH2O pressure drop (as show on below curve)


At maximum flow of 10 MMscfd, a change of 1% of full flow (0.1 MMscfd) to 9.9 MMscfd will be represented by a change of 2% of dP (from 100 inH2O to 98.01 in H2O). The following table summarized the value of flow, its corresponding pressure drop, and its dP change in percentage of full scale for 1% change of full flow at the specified flow. The less flow to be measured, the more sensitivity suffers.


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Actuated Valve Safety Factor

During designing an actuated valve, instrument engineer shall concern to valve safety factor which is a ratio between “torque produced by an actuator” to “torque required by a valve to actuate when changing position” (close to open or vice versa). Valve and actuator commonly obtained from different manufacturer, therefore it is a responsibility of instrument engineer to ensure that combination of the selected valve and actuator will operate properly and meet the safety factor specified by project.

To obtain the information simply doing the following step.

After deciding valve size, valve rating, valve material, etc., go to valve catalog and select one valve that meet the specification. Then, obtain valve torque data. Valve torque is affected by maximum differential pressure across valve (Max dP). This value is determined by process department and shall be informed to vendor. Max dP usually occurs when valve is fully close so that one side at maximum pressure while the opposite site is at no process fluid condition.

Afterwards select an actuator that can produce torque higher than the required valve torque. Note that torque produced by an actuator is affected by power source, e.g. instrument air pressure in pneumatic actuator.

Usually vendor will provided safety factor table along with quotation, following with engineering review. Below is one example of safety factor table:

(Click To Enlarge)

What is the recommended safety factor value? Many projects require 1.5 or 2. Note that safety factor which is too high could lead to miss-operation of an actuated valve.

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