Flow Measurement

Why Drain and Vent Holes are Essential for Efficient Orifice Plate Operation?

In industrial applications, orifice plates are often used to measure fluid flow by creating a pressure drop. These devices are dependable, affordable, and easy to use. On the other hand, in certain circumstances, unique design features like drain holes and vent holes are added to the product in order to improve its operation and guarantee accurate measurements. 

Orifice plate drain and vent holes are the subject of this article, which provides a comprehensive analysis of their function, design, and quality standards.

Why Drain and Vent Holes are Essential for Efficient Orifice Plate Operation?

Click here for Orifice Plate Flow and Pressure Drop Calculation Excel Tool

A drain hole is a tiny opening in the orifice plate’s lower section. It is primarily utilized in gas flow systems to enable confined liquids to avoid the plate, ensuring accurate flow readings and avoiding problems with operation.

Unmeasured flow is introduced when a drain hole is present. However, research indicates that the unmeasured flow makes up less than 1% of the overall flow if the drain hole’s diameter is less than 10% of the orifice bore. By doing this, the measurement errors are kept to a minimum.

The presence of a drain hole introduces unmeasured flow. However, studies show that if the diameter of the drain hole is less than 10% of the orifice bore, the unmeasured flow constitutes less than 1% of the total flow. This ensures that the measurement errors remain negligible.

Because of the possibility of clogging holes, it is not advised for use with slurry or unclean fluid. Other designs, such as the eccentric orifice plate, are favored in certain situations.

Wet gas streams, where liquids may occasionally flow through the system, are perfect examples of gas flows with intermittent liquid content. Their ability to let tiny particles flow through the plate through the drain hole makes them useful in systems where dirt collection is an issue.

For efficient drainage, the drain hole is positioned at the lowest point tangential to the inner pipe surface. To avoid partial or total blockage of the drain hole, the upstream pipe, orifice plate, and downstream pipe must all be properly aligned.

A vent hole is a tiny hole in the orifice plate’s upper section. In order to avoid accumulation of gases and measurement errors, it is employed in liquid flow applications where entrained gas bubbles must escape.

 During liquid flow operations, gas bubbles that can get trapped on the upstream side of the orifice plate might escape through the vent hole. This keeps flow measurements accurate.

Just like the drain hole, the size of the vent hole has a big impact on accuracy. Errors are kept to a minimum when the vent hole diameter is less than 10% of the orifice bore, meaning that unmeasured flow is usually less than 1% of the total flow.

Due to the high danger of blockage, vent holes are not advised for services using slurry or dirty fluid. An eccentric orifice plate is a superior option in these situations.

To enable the efficient removal of gas bubbles, vent holes are placed at the highest point tangential to the pipe’s inner surface.

Click here for Orifice Sizing Guidelines & Rules Associated With Selection

The design and sizing of drain and vent holes are governed by several industry standards. These guidelines ensure that the flow measurement remains accurate and the operational integrity of the system is maintained. The Primary Standards are:

Comprehensive guidelines for fluid flow measurement using pressure differential devices such as orifice plates.

Standards for orifice metering of natural gas and hydrocarbon fluids.

Requirements for small-bore precision orifice meters.

Provides specific guidelines for the inclusion of drain and vent holes in concentric square-edge orifice plates. The standard suggests that the diameter of these holes should not exceed 10% of the orifice bore diameter. Correction formulas for bore diameter adjustments are also provided.

In order to solve particular flow measurement issues in gas and liquid systems, drain and vent holes in orifice plates are crucial design elements. Although they provide useful answers, appropriate location, sizing, and design are necessary for them to be effective. Accurate measurements and dependable functioning are guaranteed when standards like ISO 5167 and ISO/TR 15377 are followed. Computational modeling is being refined through ongoing research and development, opening the door to better industrial applications.

  • Ideal drain/vent hole diameter should be <4% of orifice bore to minimize errors.
  • Optimal angular orientation between impulse lines and drain/vent holes is 180° to reduce discharge coefficient shifts.
  • Holes <10% of bore diameter lead to unmeasured flow <1%.
  • Recent studies show smaller hole diameters yield better measurement accuracy.
  • The most efficient drain holes are found in gas systems with little liquid. Their application in moist gas streams guarantees the preservation of measurement precision and operating efficiency.
  • In order to prevent obstruction and guarantee efficient drainage, proper alignment and sizing are essential.
  • For liquid systems that contain entrained gas, vent holes are perfect. Gas buildup that can impair flow measurements is avoided by them.
  • Precise positioning and dimensions provide maximum performance and a few errors.
  • In applications involving single-phase flow, both vent and drain holes are less efficient. Additionally, they have a tendency to plug into slurry or unclean services, which restricts their use.

Click here for Classification of Orifice Plates & Pressure Tappings in Orifice Plate

In scenarios where traditional drain and vent holes are unsuitable, alternative solutions can be employed to address the challenges of liquid or gas accumulation and ensure accurate flow measurements. These alternatives include:

Feature an offset bore to prevent liquid accumulation in gas flow or vice versa. Ideal for pipelines with wet gas or liquid carryover, preventing fluid buildup and ensuring accurate readings.

Designed with a semicircular edge to bypass fluid or gas, minimizing blockages. Common in industries like wastewater treatment, mining, and pulp and paper, where solids are present.

Have a rounded edge at the bore to improve flow and reduce turbulence. Best for low Reynolds number conditions and abrasive fluids like heavy oils, ensuring precise measurements.

Click here for Different types of orifice plates

Offer a conical shape to reduce flow disturbances and minimize pressure loss. Suitable for high-velocity flows, often used in chemical processing and power generation to ensure reliable measurements.

The drain hole provides a liquid bypass, allowing liquids in a gas stream to pass through the plate. This solution is cost-effective for measuring gas with very low liquid content, but may not be as accurate in single-phase flow compared to standard designs.

Common types of orifice plates include Concentric, Segmental, Eccentric, Quadrant Edge, and Conic Edge. Each requires empirical calibration for accurate measurements.

The primary difference lies in their usage: a restriction plate is designed to overcome pressure by increasing fluid velocity, whereas an orifice plate is used to measure flow rate. Additionally, a restriction plate is typically thicker than an orifice plate.

Click here for Orifice Plate Flow Rate Calculator

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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