In reviewing a system recently, they found a pilot-operated pressure regulator whose main valve was completely insulated.
Why is this incorrect? Very simply:
· A pilot-operated pressure regulator is installed to reduce steam pressure and to maintain a desired pressure as downstream load requirements fluctuate.
· The smaller pilot valve directs fluid (steam) to the bottom side of the main valve diaphragm which is housed in the diaphragm chamber ("bell").
· When the pressure on the bottom side of the main valve diaphragm overcomes the opposing pressure, the main valve opens.
· As the downstream steam delivery pressure changes, steam pressure is fed back to the pilot via the control line.
· The pilot then adjusts the pressure against the main valve diaphragm - causing the main valve to either open or close in order to maintain the desired downstream pressure.
Why, in this scenario, is the installation incorrect?
When the bell portion of the main valve is uninsulated, as steam is directed by the pilot to the bottom side of the main valve diaphragm, the steam condenses and the area is filled with condensate. Being non-compressible, condensate - as opposed to steam - pushes against the diaphragm providing a considerably higher regulation accuracy. Also, if the bell is insulated, it is more likely that condensate can flash (become steam again) causing an increase in volume and, consequently, pressure. The increased pressure will flex the diaphragm - opening the main valve when it is not desired (affecting accuracy) and/or possibly rupturing the diaphragm (affecting reliability). For these reasons, the bell portion of the regulator should not be insulated.
How is this corrected?
The regulator bell must not be insulated. The end result is improved regulator accuracy and reliability.
Pertinent Steam System Best Practices:
· DO NOT insulate the diaphragm bell portion of pilot-operated pressure (and temperature) regulators.
In reviewing another system, they found a blowdown valve installed at the bottom of a drip leg.
Why is this incorrect? Very simply:
· Drip legs are installed to collect (and remove) condensate throughout the steam system. Properly designed drip legs help to prevent water hammer, low steam quality, erosion, and corrosion.
· Generally speaking, drip legs should be installed in the piping at low points, changes of direction, upstream of any valve that can close, at regular intervals in straight piping, and at the ends of mains.
· Besides condensate, drip legs also collect debris - corrosion materials (pipe scale and rust), weld slag, etc. A blowdown valve is required to enable periodic removal of the debris.
Why, in this scenario, is the piping incorrect?
When installed at the bottom of the drip leg, debris will settle in the valve and it can become partially or completely plugged. This can result in non-use or attempts at unplugging the valve while pressurized. Non-use can result in a clogged steam trap and unplugging attempts (many times with a welding rod or screwdriver) can result in injured personnel. These are both safety issues.
Experience has shown that when debris settles on the valve seating surfaces, upon actuation the debris can damage the seating surfaces and cause leakage. This is a reliability issue.
How is this corrected?
The blowdown valve should be installed off of the side of the drip leg, as illustrated (below). Mounting as such will help to avoid settling of debris in the valve. When blowing-down, the venturi effect will enable removal of the debris from the bottom of the drip leg. This orientation will help to prevent valve pluggage and will protect the integrity of the valve's seating surfaces. The end result is improved safety and reliability.
Pertinent Steam System Best Practices:
· Ensure drip legs are properly designed - including placement of the blowdown valve off of the side of the drip leg.
· Evaluate all drip leg steam trap stations frequently and regularly.
· Provide regular, recurring steam system training for personnel.
· Specify continuous, at-a-glance monitoring of steam temperature to ensure there is no subcooled condition. ("No subcooling, no waterhammer." - Wayne Kirsner. P.E.)
For training your people get ahold of Gleason Steam.
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