Solving Instability Issues In Commercial And Industrial .

White PaperD352697X012April 2018Solving Instability Issues in Commercial andIndustrial Natural Gas System ApplicationsNoise and vibration can be some of the largestdrawbacks of using regulators in any gas system.Natural gas transmission and distribution companiesoften go to great lengths to reduce the noise andvibration made by regulators and to prevent themfrom causing an inconvenience for their customersand people living in the surrounding area. Typically,a regulator will exhibit some noise as a function ofthe reduction in pressure. This can be evident inlarge pressure reductions like in City Gate stationsor Transmission applications. In these applications,several regulator brands offer silencing devices todampen the noise, like the Fisher Whisper Trim .However, there are times when regulators incommercial and industrial applications will becomenoisy even in installations with small pressurereductions. This whitepaper will cover the backgroundbehind these types of applications and potentialsolutions that can address many of these issues.

White PaperD352697X012April 2018 – Page 2Types of Instability and Contributing FactorsRegulators exist in a complex natural gas systemwhich includes the upstream and downstreampiping and everything in between. A regulator is notinherently unstable but may be the most likely placein a gas system where an instability condition maymanifest itself. This is because a regulator is one ofthe few system components with moving parts whichcan vibrate at the system’s resonance frequency.Resonance occurs when the set of a vibrating system’scharacteristics cause it to oscillate at much higheramplitudes in a specific frequency range.The system can become unstable by one or more ofthe modes listed below:Harmonic InstabilityHarmonic instability is the most common mode ofinstability that regulators exhibit. This condition iscaused by a collection of system characteristics duringnormal operation which results in a “humming” or“buzzing” sound emitting from the regulator dueto the vibration of the diaphragm or other parts.Harmonic instability is typically a system issue but canbe exacerbated by either hunting or feedback, whichwill be explained in more detail in later sections.Every system and component in a system has a naturalfrequency. With that in mind, any physical change tothe system may also change the frequency requiredto excite the regulator to a resonance condition. Forexample, a certain meter set may begin exhibiting ahumming noise at a moderate inlet pressure and flowbut stop when the flow increases. This is somethingEmerson engineers have seen in lab testing.Emerson engineers have been told by field techniciansthat they have occasionally noticed a regulator beginto hum after removing the closing cap for a set pointadjustment and would stop once the cap was replaced.This is because the regulator operating very close to itsresonance frequency, something as small as removingthe closing cap may be enough to cause the regulatorto hum or buzz. In a steady-state condition, theregulator was likely on the brink of instability and byremoving the cap, changed the system characteristicsjust enough that it became unstable.1. Earney, W.H. 1995. “Causes and cures of regulator instability”. United States.ABACORRECTBINCORRECT - PRESSURE MAY BE LOWER IN THESMALLER PIPING DUE TO HIGHER VELOCITYFigure 1. Regulator Control Line Placement in SwagesMany researchers in academia have developed modelsto study the stability characteristics of regulators(1).One such study analyzed the effect of large vent pipingon system instability. The researchers could adjust thesize of the regulator’s vent piping to induce instabilityin regulators, another factor to keep in mind duringsystem design.Hunting“Hunting” is a condition in which a regulator’s outletpressure slowly fluctuates on either side of the setpoint. In this condition, the regulator will “hunt” fora desired pressure but certain factors may prevent itfrom quickly reaching equilibrium.Hunting can occur as a result of several factors causingturbulent flow in the system: Pressure RegistrationA poor regulator setup may lead to increasedprobability of regulator instability. In externalregistration applications, the placement of thecontrol lines makes a significant impact on theoperation of the regulator. If the control lines areincorrectly placed in the system, the regulatorwill exhibit hunting to try to match downstreampressure to a set point using an erroneous referencepressure. Common sources of turbulent flow ingas systems include elbows, swages, valves andmeters. Placing a pressure tap too close to thesecomponents may negatively affect the operationof the regulator and may contribute to instability.Figure 1 shows an example of a correct and incorrectplacement of these control lines. Control line

White PaperD352697X012April 2018 – Page 3diameter is also a factor to consider. If the diameterof the control line is too small, it may trigger aninstability condition because it would slow regulatorresponse time. Piping CharacteristicsSmall volume piping downstream of the regulatorcan exacerbate pressure transients. In cases likethese, the regulator may not be able to respondquickly enough which can slow down the regulator’sspeed of response. This may lead to erraticpressure control resulting in excessive vibration orharmonic instability. Effective Flow AreaSwages or other changes in piping diameter maycause turbulence resulting in erratic system pipingpressures. If the regulator is set up to sense pressurenear an area of turbulence, it may erratically attemptto respond to this pressure which may cause it togo unstable.Restrictions downstream of the regulator could alsocause similar problems. In large runs of downstreampiping, instability can be caused by pipingrestrictions such as orifice plates and blocking valves.This could make the downstream volume sensitiveto load changes and make pressure much moredifficult to control, resulting in hunting. Flow CharacteristicsFlow characteristics may also be a root cause ofsystem instability. High flows and rapidly changingflows can all be contributing factors to erratic localpressures in a system. The regulator may continuallyovershoot the set point as it attempts to controlpressure under these conditions.The hunting type of regulator instability can also becaused by regulator sizing and selection decisions: Regulator SizeHunting can also be caused by incorrect regulatorsizing. If the orifice is too large for the application,it will operate very close to the seat during normalFigure 2. Commercial Meter Setoperation requiring low flow. Not only does thiscontribute to seat wear over time, but the regulatorwill be more sensitive to changes in demand. Thiscauses the regulator to overcompensate and takemuch longer to achieve the desired set point.Feedback MetersSome meters, such as larger turbine or rotary,meters, have operating frequencies close to those ofself-operated regulators as seen in Figure 2. Thesemeters create pulsations that can feed back into aregulator in close proximity, sometimes resonatingin the diaphragm. This may cause a “chugging”sound in the regulator. The less piping betweenthe meter and the regulator, the more likely thisphenomenon is to occur. Large diaphragm meterscan also cause this condition but at much lowerfrequencies, manifested by a chugging sound.

White PaperD352697X012April 2018 – Page 4FiredHeaterFuel GasHeaderPressureRegulatorPressureRegulatorBurner GasFUEL GASSOURCEControl ValveMinimum FirePressure RegulatorPressureRegulatorPilot GasFired Heater #2Fired Heater #3Figure 3. Industrial Boiler Train Boilers and BurnersIn certain boiler applications, Figure 3, most notablyin “pulse-type” combustion models, feedbackcan occur causing excessive regulator vibration.Combustion in these applications is sustainedthrough a series of pressure swings which draws theflow of the gas and air mixture into the system tosustain the process. These pressure fluctuations arenecessary for the boiler to operate correctly but mayalso cause the regulator vibration.Instability DrawbacksDepending on where the regulator is installed, thiscondition may range from somewhat innocuous (aregulator installed in a basement far away from anyonewho may hear it) to a large annoyance (a regulatorinstalled in an apartment building or school). The noiseor vibration may be so problematic that the end usermay have to redesign or move the system, exposingthem to a significant additional cost.Although the operation of the regulator is typicallynot affected in an unstable system condition, if thecondition is not addressed, the excess vibration mayfatigue and eventually damage the regulator in moresevere conditions. To correct this instability, it maybe necessary to change one or more aspects of thesystem to prevent resonance and feedback fromoccurring. Depending on the change required, this1. Earney, W.H. 1995. “Causes and cures of regulator instability”. United States.may be a large expense for the end user but is still lessthan the cost of system downtime if the regulator wereto become damaged.What Can Be Done?In the paper, Causes and Cures of Regulator Instability(1),W. H. Earney points out that, while manufacturers dothe best they can to prevent unstable conditions in mostinstallations, since an infinite number of installations exist,there are also an equal number of combination of systemand regulator components that will result in instability.However, there are several best practices to correct anunstable condition:Regulator Sizing and SelectionWhen designing a regulator, manufacturers attemptto balance pressure control with instability by carefulconsideration of the spring selection, diaphragm areadesign and amount of travel by the valve plug.The manufacturer can change the diaphragm area ofthe regulator but more realistically, the customer canonly change the orifice or spring. For spring selection,Emerson recommends the lightest spring, howeverif the next heavier spring is advertised as able tocontrol the required set point, then it can be used toreduce the sensitivity of the regulator. Emerson also