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Maximize Productivity Using Your Steam Pressure Regulator

To get maximum productivity from any steam system requires delivering the steam at its most energy efficient pressure.  Reducing steam pressure can save money because process efficiency is increased.  Reduced steam pressure protects vulnerable equipment, cuts fuel costs, and provides greater security from injuring personnel and damaging products.

  To size a steam pressure regulator, the following information is required:

1.   Inlet supply pressure.  This is not necessarily the boiler pressure.  Due to line losses, the boiler pressure may differ considerably from the steam pressure taken immediately ahead of the regulator.

2.   Outlet steam pressure desired.

3.   The steam capacity required expressed in pounds per hour.

The correct size is most important for satisfactory operation.  In general, the smallest size regulator that will handle the peak load is the best one to use.  If a regulator is oversized, wire drawing of the seat will result, especially as the load drops off.  If a regulator is too small, steam flow will be inadequate to handle peak load demand.

It is good practice to limit the pressure reduction ratio to no more than 10 to 1.  If the load is constant, ratios as high as 20 to 1 may function, but noise will be a problem.  High turndown ratios also decrease service life.  It is always best to use two regulators in series if the turn down is going to exceed 10 to 1.  While the initial cost of two regulators in series may be more than one regulator, the longer life, less maintenance, and better control of the flow will soon recover the cost difference.

In steam systems where load demands fluctuate over a wide range, parallel multiple regulators perform better than one regulator.  Multiple regulators with combined capacity to meet maximum load will reduce maintenance and lifetime cost.

Any reducing valve must be capable of meeting the maximum load and modulating down to zero load when required.  The amount of turndown any valve can handle is limited.  If the low load condition represents 10% or less of the maximum load, two valves should be used in parallel.

To the three previously mentioned pieces of information needed to size a regulator, the following should be added:

Is the load consistent or does it vary?  If it varies, what is the range of variance?  The accuracy of the information gathered would greatly influence the choice of product, and consequently a system that works well — or works poorly if too many specifications are guessed at.

Most customers will be able to supply the three basic specifications to size a regulator:  inlet pressure, outlet pressure, and maximum flow rate, but will have a problem with the split between loads.  If the customer knows there will be a large variation in load, but does not know what the actual low load is, it is accepted practice to simply select valves with capacities of 1/3 and 2/3 of the maximum load.

The following examples will illustrate the selection process.  (All regulators used in the examples will be Sarco Series 25.  At this writing, Climatic Control Company stocks Sarco.  Sarco is an excellent pressure-reducing valve, known for long life and high quality.  The 25 is a “dead end” service regulator, meaning it will shut off when there is no flow.  All selections are made using Sarco’s 1993 Product Catalog, pages 24 to 36.)

Example I 

Inlet pressure:  100 psig

Outlet pressure:  30 psig

Capacity:  5000 lbs./hr

The load is constant. 

The turndown ratio is well within the 10:1 parameter, so series regulators are not needed.  The load is constant, so parallel regulators are not needed.  The current Sarco 25 body can be selected directly from the Sarco capacity chart.  See Figure 1.  Enter the chart on the left under the “inlet steam pressure psig” column.  Under “outlet steam pressure psig” find the appropriate pressure.  In this case, it is the “0-48” figure.  Go across, to the right, until a capacity matches or exceeds the pounds per hour specified.  In this example, a 2" 25 body is required.  To the 25 body a pressure pilot must be added.  This is the “25-HEAD” pressure pilot.  Three outlet pressure springs are available and are color-coded.

Yellow Spring:  3 to 30 psig

Blue Spring:  20 to 100 psig

Red Spring:  80 to 290 psig

Figure 1.

The “25-HEAD”, in Climatic Control Company stock, is equipped with a yellow spring.  It is not good practice to use any spring at either the high or low end of the range, so the 25-HEAD should be equipped with a blue spring instead of the yellow spring.

Example II

Let’s take Example one and change the outlet pressure to 5 psig.  The turndown ratio is now 20:1, twice as much as the recommended 10:1 turndown ratio.  This situation should be taken care of with regulators in series.  It is best to try and take most of the drop through the first valve and the second valve used for “fine control”.  But to keep costs reasonable, one must look carefully at what happens if too much turndown is taken through the first regulator.  In every case of series regulators, he second, or downstream, regulator will be larger, or at least the same size, as the first or upstream regulator.  This is because capacity has to be maintained at lower pressure drop through the second regulator.

If we use the first regulator to drop the pressure to 15 psig, the regulator will be a 2" regulator.  But the regulator to take 15 psig to 5 psig would need to be a 6" regulator to maintain capacity!  If the first regulator is used to drop the pressure to 50 psig, the regulator will only have to be 2” in size. The second regulator then can be utilized to take the 50 psig to 5 psig (within the 10:1 turndown ratio) and results in a 2-1/2" regulator, much more economical than a 6" regulator.

Certain special piping practices should be observed in series applications of regulators. (Steam piping practices and pipe sizing is beyond the scope of this Info-Tec.  Mention is made of special piping to alert the reader that the total installation must be looked at.)

Example III

Using the specifications from example one again, but now the customer informs us of large load variations.  The customer is unable to supply the low load figure, just that there is a “big difference” in demand.  Paralleling regulators will result in the best control for this application.  Since the customer could not be specific in supplying the low load capacity, selecting the valves by using 1/3, 2/3 split of capacity ratings will be used.  This results in one valve sized for a maximum capacity of about 3300 lbs./hr and the other valve 1700 lbs/hr.  The 25-1-1/2" 100 to 30 psig is rated for 3460 lbs/hr, and a 25-1" is rated at 1815 lbs/hr.  These valves should be installed in parallel as in Figure 2.  One valve’s outlet pressure should be set at 29 psig and the other at 31 psig  If the smaller valve is set at 31 psig, this valve will control from 0 to about 1800 lbs/hr at 31 psig  As the load increases, the pressure will drop and the valve set a 29 psig will begin to open to supply the load.  There are applications where the load does not fall below the minimum capacity of the larger valve very often.  It should then be best to set the larger valve at 31 psig and supplement the flow through the smaller valve set at 29 psig on the few occasions when extra capacity is required. 

Figure 2.

Figure 3 and Figure 4 illustrate typical reducing valve stations.  Note all the accessories that should be included, along with the reducing valve(s): gauges, traps, relief valves, shut-off valves, and strainers.

So far, all the discussion has been on the 25P version of the Sarco regulator.  “P” stands for a pressure pilot with outlet pressure adjustment done by tensioning a simple spring.  To change the outlet pressure within the spring range, an adjusting screw must be manually turned with a wrench.  If remote set point is desired and air is available, a “PA” pilot can be used.

Figure 3.

Figure 4.

“PA” is a “Pressure Pilot - Air”.  There are three downstream steam pressure ranges of PA pilots available.  PA is available from 3 to 35 psig with a ratio of 1 to 1.  This ratio means for every pound of air pressure change, the downstream steam pressure changes one pound.  PA4 is available from 10 to 120 psig with a  ratio of 1 to 4.  For every pound of air pressure change the steam pressure changes four pounds.  PA6 is available from 20 to 225 psig with a ratio of 1 to 6.

Figure 5.

Figure 5 is a chart to select the proper air pilot.  To use the chart, draw a vertical line up from the maximum air pressure available.  Draw a horizontal line to the right from the maximum downstream pressure required.  Where the lines intersect will be within a section of the chart showing which PA pilot to use.  Example:  Select a pilot to regulate downstream steam pressure to 60 psig maximum.  20-psig-air pressure is available.  Draw a vertical line up from the 20-psig line.  Draw a horizontal line to intersect this line from 60-psig-steam pressure.  The intersection of these two lines falls within the portion of the chart covered by the PA4.  Use a PA4.

Other pilots available from Sarco are temperature and electric pilots.  Pilots can be attached to the basic 25 body in multiple configurations, combining actions for desired operation.

Temperature pilots are available in ranges from 60oF to 260oF, with twelve different styles of bulbs and wells on two standard capillary lengths of 8 feet or 15 feet long.  Other lengths are available on special order.  Temperature pilots work by sensing the temperature of the controlled medium and modulating the steam flow through the 25s main valve body to maintain set point.  No electricity is required.  The temperature pilot does not make a pressure-reducing valve out of a 25!  To have a 25 be both a pressure and temperature valve requires the use of a temperature pilot and pressure pilot.  This is a common combination, used extensively when using high-pressure steam to heat water for domestic use or space heating, when using a heat exchanger.

An electric pilot can be added to any combination of 25’s to provide on/off operation.  The electric pilot is a solenoid valve when de-energized causes the 25 main valve to close, regardless of what any other pilot may be signaling.  When energized, the electric pilot returns control to any other pilot or pilots on the 25 body.  If there are no other pilots, then the 25 is simply opening and closing, on off, acting like a simple solenoid valve.

Since there are so many possible combinations and components available to complete the Sarco 25 valve, Sarco has a matrix to help select the needed parts and correctly price them.  See Figure 6.

Figure 6.

Pages 10 to 16 of Sarco’s “List Price Schedule”, dated June 1, 1994, is not just a price sheet, but a catalog of the components of the 25.  Sarco’s “Product Catalog” only gives an overview of the 25 product line, while the price schedule gives the ordering information for the 25 line.

Sarco’s high quality, and consequently initial high cost, can be justified because of long service life and low maintenance.  Over time, the installed cost of a Sarco 25 will be less expensive than another cheaper brand of valve.

Unfortunately, occasions will arise where initial first cost is the most important consideration.

For those occasions, Climatic Control Company handles competitively priced lines of steam pressure reducing valves manufactured by Watts Regulator Co. and Hoffman.

The Watts is strictly a pressure-reducing valve.  No pilots are available.  The series 127 is a dead end service regulator with high capacity capability.  The Watts 152A and 252A regulators are small capacity regulators and are not tight seating for dead end service unless ordered special.  A “T” must be added to the model number to get a dead end service regulator.  The 252A is identical to the 152A, except the 252A is furnished with a special composition bronze body for supply pressures in excess of 200 psig.  The 152 and 252 are equipped with a sensitivity adjusting screw that can be used to eliminate vibration noises that may occur during critical flow conditions.

Note:   Watts sizing charts and literature implies that turn down ratios as high as 50 to 1 are acceptable.  While the regulator might function in ratios in excess of 20 to 1, noise would be excessive and service life greatly shortened.  As has been discussed, it is good practice to limit turn down ratios to 10 to 1. 

To size a Watts regulator, a co-efficient chart is used.  See Figure 7.  Under the known initial pressure and opposite the required reduced pressure, select the figure shown.  Divide the amount of steam required by this figure.  The answer is the valve co-efficient.  Select a valve whose co-efficient is equal to or next larger than this answer.

Figure 7.

Example: 

Inlet pressure = 100 psig

Outlet pressure = 45 psig

Flow rate = 275 lbs/hr

Under 100 and opposite 45 is 5950

The co-efficient is 275/5950 = .046

In Table “A”, the nearest co-efficients are .065 for a 1/2" type 127, or .85 for a 3/4" type 152A-252A.  If price is the overriding factor, use the 152A.  If quality and long life is considered, use the 127.  (The 127 is about three times the cost of a 152 size for size.)

At the present time, Climatic Control Company does not stock the ITT Hoffman line of regulators, but it is readily available to us.  The Hoffman 2000 series of regulators are, like Sarco, a complete line of regulators with spring, air, thermostatic, and electric pilots. 

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