DESIGN & SAFETY HANDBOOK|
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PRESSURE REGULATORS: MAINTENANCE
|Regulator maintenance is an important part of maximizing your system’s performance and extending the service life of system components. A maintenance schedule is the frequency at which recommended maintenance operations should be performed. Adherence to a maintenance schedule should result in minimizing downtime due to regulator failure as well as enhancing safety in the work area. Regulator service defines the gas service in which the regulator is installed in terms of its corrosive nature. There are three categories: noncorrosive, mildly corrosive and corrosive. Establishing the category a regulator fits into can be difficult. Consult your Scott Representative.|
Recommended Schedule - This schedule should be used as a general guide. Be sure to follow the manufacturer instructions supplied with your regulator.
1 – 2 years**
3 – 4 years
1 More frequent overhaul or replacement may be required for regulators installed in a corrosive ambient environment.
* If diaphragms are neoprene or another elastomer, they may dry out and require more frequent replacement.
** If regulators are not properly installed and used, or if a poor grade of gas is used, or if purging is not properly done, overhaul and/or replacement may be required more frequently than indicated.
† For regulators used in toxic or corrosive gas applications, care should be taken to ensure proper precautions are followed, as recommended by Scott.
NA Not applicable.
Leak Check - With a regulator under pressure (both high and low pressure side) check all connections for leaks using a gas leak detector (Scott Model 46-B Series) or Snoop®. If a leak is detected, shut down the gas source, reduce pressure to atmospheric, and tighten or redo the leaking connection. Retest. If leak persists, contact Scott. Warning: If the connection must be redone (i.e. to replace a compression fitting), regulators used on toxic or corrosive gases must first be purged with an inert gas such as nitrogen. Consult Scott or the regulator manufacturer for specific purging instructions.
How to Perform Dilution Purging - The most effective means of purging regulators and connecting lines is the dilution purging method.
Step 1: Attach the regulator to the specialty gas cylinder. A tee with a valve on the side branch should then be located in the line between the regulator and the instrument(s). This branch should be connected to a safety vent while the main trunk runs to the instrument(s). The tee should be located close to the instrument so that the connecting line between the regulator and instrument is also purged.
Step 2: Adjust the regulator to the fully closed position. Then, close the safety vent valve and the valve at the instrument and open the valve on the outlet side of the regulator.
Step 3: Open and quickly close the cylinder valve to pressurize the inlet side of the regulator to cylinder pressure. It is necessary to quickly close the cylinder valve after each cycle to keep downstream contaminants from entering the cylinder until the regulator is fully purged. Mounting the regulator on a single-station manifold that incorporates a check valve in the pigtail will eliminate this problem.
Step 4: Adjust the regulator to establish an appropriate delivery pressure and open the vent valve to bleed off the regulator pressure.
Steps 2 – 4 represent one purge cycle. This cycle should be repeated three to five times to ensure that the regulator and connecting line are both properly purged.
|Creep Test - Regulator creep is a phenomenon in which delivery pressure rises above a set point. Creep can occur in two ways. The first is due to changes in the motion of the regulator springs when gas flow is stopped. When flow has stopped, the springs must move to a new position of equilibrium, causing a slight increase in delivery pressure. This type of creep may be thought of as the opposite of droop. The second and more insidious type of regulator creep is caused by foreign material being lodged between the poppet and seat, thus preventing tight shut-off. The result is that inlet and delivery pressure can equalize across the regulator, exposing all tubing and instrumentation to the inlet pressure. Regulator creep as a result of seat failure due to foreign material is the single most common cause of regulator failure. In order to prevent costly damage to the gas delivery system and the instrumentation it serves, care must be taken to ensure that regulator connections are capped to protect against ingress of dirt or foreign material. Tubing should also be flushed or blown clean to remove any foreign matter. A pressure relief valve should be installed downstream of the regulator as additional protection against creep. |
To creep test, isolate the downstream side of the regulator by closing the regulator outlet valve, instrument valve or process isolation valve. Close the regulator by turning the adjustment knob counterclockwise until it reaches stop or rotates freely. Slowly turn on the gas supply. When the regulator inlet gauge registers full cylinder delivery pressure, shut off the gas supply. Turn the regulator adjusting knob clockwise until delivery pressure gauge reads approximately half of scale (i.e. 50 psi (3 bar) on a 100 psi (7 bar) gauge). Close the regulator by turning the adjustment knob counterclockwise until it rotates freely or reaches the stop. Note the reading on delivery pressure gauge. Wait 15 minutes and recheck the setting on delivery pressure gauge. If any rise in delivery pressure is detected during this time, the regulator is defective. Remove and replace.
Regulator Purging - Regulator purging is not always given the attention it deserves in the use of both high-purity gases and calibration gases. It is easy to understand that special precautions are necessary when using pyrophoric, toxic, corrosive, flammable or oxidizing gases. These gases require special safety precautions or special gas handling equipment. The results obtained from the use of nonreactive gases and mixtures, however, can also depend upon how the gas delivery system is prepared. In order to maintain cylinder integrity and obtain the best results possible, the end user should purge all regulators. The connecting lines and the purging procedure influence the quality of the end gas. Purging of regulators is often either not done at all, or is done by simply allowing an arbitrary amount of gas to flow through the regulator. But, there is a shortcoming to this method. In virtually all regulators, there are internal dead pockets that tend to hold contaminants. These pockets tend to be unaffected by the flow of purge gas. Better results will be achieved by alternately pressurizing and depressurizing the regulator with the purge gas. This is called dilution purging.
Overhaul - All regulators should be removed from service periodically and returned to the manufacturer for inspection/overhaul as appropriate (see Regulator Maintenance Schedule).
Replacement - Regulator failure that warrants regulator replacement will vary considerably based on conditions of use. However, once the life expectancy of a regulator has been exceeded, it should be replaced to prevent failure. Contact your Scott Representative to determine the life expectancy of your particular regulator model.