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CONTINUED | 4 <br />Gas pressure regulators are feedback control systems <br />driven by the pressure differential across the <br />diaphragm and the case spring. When gas flow on <br />the low-pressure side of the regulator causes a <br />pressure drop, spring force in the regulator case <br />pushes on the diaphragm and opens the valve to <br />increase gas flow to maintain the set pressure. <br />The dynamic pressure maintained by the regulator <br />decreases slightly as gas flow rate increases <br />(Figure 2). This phenomenon is known as pressure <br />droop or, more simply, “droop”. Regulator <br />manufacturers design products to minimize pressure <br />droop while still maintaining regulator stability for a <br />given gas flow rate. <br />Regulators tend to exhibit the best stability and <br />response time when they operate near the middle of <br />their proportional band. Selecting a regulator with a <br />published maximum gas flow of approximately 1.5 <br />times the full-load gas flow required by the generator <br />avoids operation very close to the fully open or fully <br />closed position, minimizing the probability of <br />unstable operation. A regulator that is too large, <br />capable of flowing several times the maximum gas <br />flow required by the generator, will operate very <br />close to its fully closed position which may also <br />result in unstable operation. <br />2.4. Spring Rate, Accuracy, and Response Time:4 <br />The regulator spring provides the force required to <br />open the regulator valve and maintain the desired <br />operating pressure. There may be more than one <br />spring covering a desired operating pressure. Spring <br />selection plays a role in regulator accuracy and <br />response time. <br />In general, using the lightest spring rate (a blue spring <br />from the prior example referencing Figure 2) that <br />achieves the desired operating pressure will provide <br />the best accuracy, minimizing pressure droop across <br />the range of expected gas flow rates. However, a <br />response that is “too fast” can introduce oscillation <br />and instability. If instability is experienced during <br />operation, moving to the next higher spring (a green <br />spring from the prior example referencing Table 1) that <br />includes the desired operating pressure is one <br />potential method to mitigate oscillations. <br />2.5. Orifice size: For regulators where various <br />orifice sizes are available, select the smallest orifice <br />that will provide approximately 1.5 times the <br />maximum gas flow required by the generator. <br />Selecting an orifice that is significantly larger than <br />necessary will result in the valve operating very close <br />to the seat (nearly closed) and may result in pressure <br />instability, increased seal wear, or audible noise from <br />the regulator. <br />2.6. Lockup or hard shutoff: A regulator with a <br />lockup or hard shutoff feature must be used. Lockup <br />is the pressure above the regulator setpoint that is <br />required to shut the regulator off tight so no gas <br />flows. Typically, the lockup pressure is 1"-3" W.C. <br />above the dynamic pressure setpoint measured when <br />a small volume of gas is flowing (i.e. no-load running <br />condition on the generator). The lockup feature <br />prevents the low-pressure side of the regulator from <br /> <br /> <br /> <br />Figure 2: Pressure droop characteristic of a typical <br />direct-operated regulator. Courtesy of Emerson-Fisher <br />Natural Gas Application Guide. <br />P 2 <br />LOCKUP <br />SET-POINT <br />DROOP <br />(OFF-SET) <br />WIDE-OPEN <br />FLOW <br />Natural Gas Supply System Design Guide for Generac Industrial Spark Ignited Generators <br />Docusign Envelope ID: 2F58BEE5-DDBE-42EB-8DFA-5E2508488B04