Orifice Sizing Guidelines & Rules Associated With Selection

In every process industry, an orifice plate is a powerful equipment flow-measuring device. Since the cost of operation of an orifice is very less compared to other flow meters.

Sizing of an orifice plate

Some sizing factors influence the accuracy of an orifice plate such as

1. Differential Pressure across an orifice plate

• As we all know that the Differential Pressure across the orifice plate is directly proportional to the square root of the flow rate of the fluid but not linear.
• The DP measurement error increases at a lower flow rate and reduces the accuracy of an instrument. 
• At this instant, an error is introduced at a low flow rate through low full-scale DP corresponding to a high flow rate,
• But, at a high full scale, the accuracy of an instrument is achieved with higher pumping or higher energy cost.
• Here some standard DP ranges like 50 inches, 100 inches, & 200 inches of water column are considered.  But during sizing 100 inches is selected instead of 50 inches, and 200 inches is selected instead of 100 inches of water column.

2. Higher flow rate for the particular application

• The flow measuring system maintains a better accuracy level at the highest flow rates.
• But, maximum flow may introduce an error at lower flow rates, based on actual conditions an accurate high flow range must be selected to obtain better accuracy results.

What are the guidelines considered for sizing of Orifice Plate?

General guidelines for sizing of Orifice Plate

• Inspect the Sizing Standard: it must be ISO 5167; AGA Report No-III; MFC-14M
• Orifice Flange standard: ASME B 16.36. For a line size greater than 2 inches, Orifice plates are fixed by using appropriate flanges, Integral Orifice: orifice for a pipeline size equal to or less than 1.5 inches.
• D-D/2 tapping: This type of tapping is used for a size greater than 14 inches. Flanged tapping lesser than equal to the 14-inch line size
• Flow Range-ability of orifice ratio for closed loop 1:4 and 1:5 for open loop.
• The range-ability of the orifice plate is 4.5 or 45000/10000, So the orifice plate can only be used for range-ability less than 5.0.
• Where flow range-ability lies between 5:1 to 10:1, for this Differential pressure transmitter Dual types are recommended to achieve a higher accuracy level.
• The  square-edged concentric orifice plate is considered a default instrument
• The quadrant-edged orifice is considered for flow service for Reynolds number below 10000.
• The conical entrance orifice is considered for flow service for Reynolds number below 2000.
• For specific flow applications, an Eccentric type orifice or Segmental type of orifice plates are employed 
• Orifice plates for various fluid types.
• For fluids that carry gases, or gas carries liquids Eccentric type orifice is selected.
• For fluids of high solid concentration or Light slurries Segmental type of orifice plates are employed 
• Conditional Orifice Plates having multiple bores are used only for special cases.
• As per Piping Material Specification (PMS), Stainless Steel-316 or higher is used as the Material for the Construction of the Orifice Flange.

What are the Thumb Rules for Sizing of Orifice Plate?

The Thumb Rules for Sizing of Orifice Plate

• For a given orifice plate, the flow rate is directly proportional to the Square root of differential pressure.
• Pressure drop across the orifice plate is inversely proportional to the Beta ratio (β) which greatly depends on the size of the orifice bore.
• Pressure drop across the orifice plate is high for lower Beta ratio (β) for smaller orifice bore
• Pressure drop across the orifice plate is low for higher Beta ratio (β) for larger orifice bore
• In some cases, measurement of low differential pressure (40mmH2O) through a DP type      transmitter may be harder.
• To achieve better accuracy with low-pressure drop across the orifice plate, a DP-type transmitter of a high range is recommended.
• To satisfy Accuracy and Permanent Pressure Loss (PPL), a particular arrangement is essential for selecting a calibration range.
• The Permanent Pressure Loss is directly proportional to calibrated Differential Pressure. 
• If and only if the calibrated DP is high, the Permanent Pressure Loss is larger, and vice versa.
• Generally, the maximum allowable Permanent Pressure Loss ranges between 0.1 to 0.2 bars.
• Approximate Permanent Pressure Loss at

Calibrated Condition = (1 – Β^1.9)* Calibrated Differential Pressure. 

Actual Condition = (1 – Β^1.9)* Actual Differential Pressure.

• But the flow range-ability of almost all DP-type transmitters is 100:1.

Practically, the usable range-ability of the DP Transmitter is about 100% (Max):10% (min) which is equal to 10:1.

• The traditional design of orifice plates used for flow range-ability in the process industry is 5:1.
• As per ISO-5167 part II, Beta ratio (β) is permitted for 0.1 to 0.7 for flow orifice. But the Beta ratio (β) for some clients is specified as 0.2 to 0.6.

The below flow chart shows the guidelines and thumb rules for sizing the Orifice.     

What happens if the Beta ratio (β) is greater than 0.7?

• The diameter of the Orifice bore is relatively large.
• Flow restriction is very less
• Pressure drop across the orifice is low
• Difficult to determine lower pressure value.
• Accuracy reduces below the expected limit
• Increases Uncertainty
• A large annular space between the orifice plate and the pipe wall distorts the flow profile for high Beta ratios. This flow disruption can cause significant pressure loss, lowering pressure recovery downstream of the orifice plate.
• Turndown ratio is the flow rate range for reliable readings. Due to the orifice plate’s nonlinear relationship between differential pressure and flow rate, a high Beta ratio may limit turndown ratio.
• High Beta ratios can increase the likelihood of cavitation, which is the formation and subsequent collapse of vapor bubbles in the fluid. 

What happens if the Beta ratio (β) is lesser than 0.2?

• The diameter of the bore of the Orifice plate is not large.
• Flow restriction is very high
• The pressure drop across the orifice is high.
• The probability & Chance of Cavitation & Flashing is high.
• Accuracy reduces below the expected limit
• Uncertainty Increases, Reduced sensitivity to changes in flow rate
• Permanent Pressure Loss surpasses normal value in the case of Process Hydraulics.
• Increased risk of blockages due to sensitivity to dirt and debris.
• Limited turndown ratio, making it challenging to measure a wide range of flow rates.

What is the actual range of the Beta Ratio of the orifice plate?

For better performance of the orifice plate, the Beta ratio is maintained in the range of 0.3 to 0.7.

What do PMS stand for in piping?

PMS stands for Piping Material Specification

What is the formula used for sizing the orifice?

• The orifice used to measure the flow rate is sized by considering the Beta ratio (β) 
• Beta ratio is defined as the ratio of the actual diameter of the Orifice plate bore (d) to the diameter of the pipe’s internal cross-sectional area (D). 
• Mathematically, the Beta ratio is given as β= d/D

What is the sizing of the orifice plate?

• Usually, in all industrial applications, these Orifice plates are fixed to the pipe size ranging between 2 inches to 24 inches.
• If the line size is less than 2 inches, there may be a chance of pressure drop through the orifice plate.

What is Reynolds’s number?

• The term Reynolds number in flow measurements is defined as the ratio of the inertial force of process fluid to its drag force.
• Here, the parameters such as flow rate or velocity, density, of fluid & diameter of pipe are considered inertial force, whereas the viscosity of the fluid is considered drag force.
• Reynolds Number   

Re= (V*D*P)/U

Where  

Re = Reynolds number

V = Velocity

P = density

D = pipe diameter

U = fluid viscosity