Pipe Reducing Factor

Holden wants to install 1" valve in 2" size pipe. But he wonder what are the reducing factor that he needs to consider. So he began to make the calculation.


Cv with piping geometric factor consideration equations are:



         
             Where Fp (R)

 

Where:

 d :           Valve inlet diameter 

D            :           Internal diameter of the pipe 

K            :           Head loss coefficient of a device, dimensionless

N2           :           constant to suit the measurement unit, 890 

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.

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)