#LinkeInsights: Base Conditions for Orifice Calculation

Flow Pressure

Orifice plates are generally used for measuring the flow rate of fluids moving through closed pipes. Base conditions, also referred to as standard conditions, comprise an absolute temperature and pressure. From this, one can calculate the orifice. Today’s #LinkedInsights feature points out the base conditions required to calculate an orifice. The feature discusses what exactly these base conditions are. Further, it points out how to get the required density at base conditions.

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Query: How to Calculate an Orifice?

Calculating an orifice requires the formula in which base conditions are temperature and pressure. In addition, the formula needs the specific gravity at the base condition. In this context, an Instrument Engineer from Hi-Tech Engineering, Korea, wanted to know the following:

  • Are the base conditions the normal ones of 1 atm, 0 deg-C (14.7 psia, 32 deg-F), are they the standard ones of 1 atm, 15 deg-C (14.7 psia, 60 deg-F), or are there other conditions?
  • Does the formula calculation require specific gravity at the base condition? If one doesn’t know the specific gravity at the base condition, how can one find the density?

Base and Standard Conditions

In response, a Senior E&I Engineer at PT Lautan Luas, Tbk, Indonesia, states that the type of base condition depends on which units your company requires. “If they are already using the standard one, then you should continue using the standard base. This will allow them to compare flow measurement.” Secondly, he offers that the base condition includes pressure and temperature. Specifically, he replies, “If you choose standard, then look for specific gravity at 1atm a 15-degree Celsius.”

Stating the similarity between base and standard conditions, an Instrument Maintenance Manager from Reliance Industries, India, commented that a base condition is also a standard condition consisting of specified absolute pressure and temperature. Hence, it should be the standard ones. “The flowing density is required in order to compensate the measured volume to quantity at base conditions,” he added.

Finding the Gravity

Continuing, a Senior Specialist Engineer at Spirax Sarco from Canada weighs in. He adds, “In the case of liquids (being virtually incompressible), specific gravity is the ratio between the weight of a substance and the weight of water @4C. The orifice plate sizing program will then calculate actual flowing density based on the flowing P & T.” Further, he stated that the standard and normal conditions usually refer to gases only.

Alternatives to an Orifice

A Sales Engineer from Krone, Netherlands offers some alternatives to using an orifice. He states, “Why use an orifice at all? Is it because you and your company are used to this? If it’s the liquid electrical conductive, then we have the ElectroMagnetic FlowMeter. However, if it is a non-conductive, we have the Ultrasonic FlowMeter. In addition, if you did want to measure mass flow, then we have the Coriolis type Mass FlowMeter,” he added.

A Measurement Engineer from Accord Energy Solutions, United Kingdom offers other alternatives. He believes that in order to measure a liquid, where the liquid is considered incompressible under ISO-5167, and SG will have little effect (close to unity), it would be better to use a Coriolis meter. “This will measure both mass flow and density, to determine volumetric flow.”

Company Conditions

An Instrument Engineer from PT Pertamina, Indonesia Highlights that a lot depends on the company one works with. He suggests that the base conditions (pressure and temperature) will be contractual/standard in the company you work with. “Usually, you can get them from the Basic Engineering Design Data or Process Design Basis. Secondly, you might not need the specific gravity at the base condition, but compressibility at base condition Zb for AGA-3 orifice. Basically, it’s the Z under Pb and Tb. You can also have a look at AGA-8 for the Z calculation, the GCM or DCM method,” he added.

Not just the company, but also the government and other contracts regulate such standards. According to an Engineering Group Supervisor at Bechtel, India, “Contract, governmental law or an agreement of the measurement by the two parties sets base pressure: AGA 3 uses 14.73 psia as the base pressure.”

Therefore, it is best to follow what companies, the government and contracts determine the base conditions to be.

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