Two Staging a Steam Regulating Control Valve

I received a call the other day from a client wanting to reduce a 120 pound steam line to 5 pounds capable of 3500 pounds per hour of steam capacity on an 2 inch steam line.

This is not an uncommon request although special considerations must be understood before just jumping in a pricing up a new control valve.

 

The first consideration is the maximum differential across the selected steam  control valve. ITT Hoffman in their 2100 series control valve suggests 2 staging for pressure drops greater than 150 psi steam pressure. Other manufactures like Spence and Spirax Sarco have similar requirements.

The second consideration is the fpm (feet per minute) of the steam going through a selected line size. The FPM is the single most forgotten part of the selection process in my experience when I have gone to jobs with customer complaints about steam regulators.

Most of the complaints range from main valve wearing out quickly to the un-earthly screaming sound the valve is emitting. Both of these complaints usually could have been eliminated from the equation if proper selection of the valve was done and by two staging the application in question.

When sizing a steam regulator the velocities for heating and indoor applications should be between 4,000 to 6,000 fpm in industrial applications the velocities should be between 8,000 to 12,000 fpm. As the steam velocity increases through the piping so does the decibel (sound) volume rating. The increase of decibels can and will cause hearing irritation.

Simply by two staging the application these problems can be eliminated. A rule of thumb is when an steam reduction application requires more than a 90 to 100 psi reduction, two stage the application. In the original application of 120 psi steam to 5 psi steam select a steam control valve that will take your initial steam pressure from 120 psi inlet and reduce it to somewhere around 70 psi outlet. Second stage another steam regulator to reduce the 70 psi steam inlet to a 5 psi outlet. Remember when doing the reductions to make sure to select valves that will ultimately give your application the necessary pounds per hour steam to run your application correctly.

The formula for figuring out the velocity of steam given a particular pipe type and size is rather complicated and finding a calculator to do it for you is not a real easy find. Here is the basic formula.

V (velocity) =  2.4 (X) W (steam flow required PPH) (X) ­V (specific volume in cubic feet per pound) divided by A (area of pipe size in square inches) Note: the V in specific volume should have a line over the V but I can’t figure out how to do that on my keyboard).

Again this is not the easist calculation to do, so if you ned help calculating your steam regulator needs, call Stromquist and Company at 1-800-241-9471 and we will help you out.

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