Control Valve operation and design criteria for beginners 3

In process systems, the final control element is normally a pneumatically actuated control valve. The control valve is subject to the hardest condition, the most expensive item and the most prone to incorrect selection.

There is no glamour in the control valve like there is in distributed control systems. Would any new engineer that graduated from distinguished University be interested in pipe, gasket, flanges, and packing and bothered working with dirt, mud, oilywater when “interfaced” with hardened stainless steel valve trim. It is also hard to hide a 400lb valve under your coat if you make mistake in sizing and selection and the valve needs to be changes.

Fig 1 Debris was found inside the control valve between the plug and seat ring

Fig 2. valve cage

But there are some reason for using valves.  they work and they are great to stand on for you to reach field mounted transmitter reached. If you are still interested, the following subject may be important for choosing the right control valve.

Control Valve Operation and Design Criteria for Beginners 2

Today, control valve can be sized easily by using software in a PC, but in the reality 83.43% of the control valve are sized incorrectly. 76.1 % are too big, 5.13% are too small, 2.2% are both (hahaha). The reason is lack of understanding about vapour pressure (which makes valve cavitate and flash), fluid property (if the service is slurry, the slurry can block the trim and everybody thinks that the valve is undersize) ,and all the things that are mentioned in the flow chart

If the valve is sized incorrectly and the valve needs to be changed, everybody will know it. And it is hard to change it insitu, not including how to perform the seat leak test when the valve had been changed.

Orifice (Differential Pressure) Flow meter Design 1

For Flow meters selection and sizing, use the orifice flow meter. You will waste some energy and money but at least you can relate to its principle of measurement. It is also requires a basic philosophy and prejudice. The most popular method of the flowmeter selection is to use an orifice unless it won’t work.

Reason for using orifice (DP flow transmitter)
Simple (but they are hard to install to get a good velocity profile)
Inexpensive (well there are cheap ultrasonic flowmeters with better accuracy and turndown ratio than DP transmitter today)
Available in various sizes, type, and connections.
Simple design, rugged, and suitable for liquids, gases, steam.

Orifice (Differential Pressure) Flow meter Design 2

Orifice plates are really hard to install. To get a good velocity profile, the mechanical specification for meter tube, the orifice flanges or fitting the differential pressure sensing taps, the upstream and downstream piping requirements must be adhere to the standard. But don’t worry, velocity profiles are only pretty pictures in engineering textbooks

Orifice Plates and holders

Orifice plates are usually installed between special flanges in a horizontal pipe run. The flanges are thicker than normal to accommodate two small bore tapings for connection to a DP cell. The plate is positioned concentrically within the flange bolt circle with the tap protruding near the top of flanges. The tab has a hole and the orifice plates must have information engraved to indicate the correct upstream / downstream orientation.

Control Valve Operation and Design Criteria for Beginners 1

Control valves are used throughout the process to control pressure, level, temperature, or flow. All control valves have variable opening or orifice. For a given pressure drop across the valve, the larger the orifice the greater the flow through the valve.

















Fig 1: Typical single-port body control valve (courtesy of Fisher)