Causes and Cures of Regulator Instability

Transcription

1 Causes and Cures of Regulator Instability

2 Introduction Gas Pressure Regulator: Controls pressure by manipulating flow Instability is improper downstream pressure control [File Name or Event] Emerson Confidential 4/22/2016 Slide 2 2

3 Regulator Overview [File Name or Event] Emerson Confidential 4/22/2016 Slide 3 3

4 The 3:00 AM Call! My Regulator is Pulsing Oscillating Buzzing Panting Cycling Pumping Chugs Vibrating Hammer Surges Hunt Floats Unstable Screams Like at Cat [File Name or Event] Emerson Confidential 4/22/2016 Slide 4 4

5 Questions to Ask Frequency of instability Amplitude of cycle: Desired set point Pressure build up, droop, cycle range Pressure conditions Piping configuration / Station Design Tap location Additional equipment Location and type of valves Regulator Type / Style Spring(s) Orifice Operating history Previously unstable Length of service Last maintenance [File Name or Event] Emerson Confidential 4/22/2016 Slide 5 5

6 System Causes System Piping A regulator by itself cannot be unstable System causes the regulator to be unstable Small volume piping Pressure changes in small volume cause erratic pressure control behavior Restricted piping A small volume piping is created by restricted piping before a large volume Orifice plates, blocked valves, and headers create flow restrictions Turbulent Piping Erratic pressure profiles within the pipe Extreme piping expansions, or contractions from swages [File Name or Event] Emerson Confidential 4/22/2016 Slide 6 6

7 Restricted Piping Restricted Piping: Downstream hand valve acting like an orifice plate can create small volume piping effect [File Name or Event] Emerson Confidential 4/22/2016 Slide 7 7

8 Load / Flow Rate Fast Changing Flow Rapid downstream flow change causes pressure decrease Regulator travels open and overshoots due to mass of internal parts causing overpressure If sustained cycling, unstable P set Overshoot travel open Overshoot travel closed Time [File Name or Event] Emerson Confidential 4/22/2016 Slide 8 8

9 Load / Flow Rate High Flows Extremely high flows cause turbulence Low Flows Requires regulator to operate too close to the seat Meters Most self-op regulators can operate near zero travel Combining system conditions with low flows can cause instability Positive displacement meter s operating frequency nearly matching self-operated regulator s natural frequency [File Name or Event] Emerson Confidential 4/22/2016 Slide 9 9

10 Valve Configuration Orifice (either valve body or removable orifice) and movable disk element (either disk or valve plug) manipulate flow Largest cause of instability is over sizing of main valve or orifice Causes valve plug or disk to operate too close to the orifice or seat Extremely sensitive to inlet pressure changes and flow demand changes [File Name or Event] Emerson Confidential 4/22/2016 Slide 10 10

11 Valve & Plug Pressure drops Can cause excess forces on valve trim affecting operation Follow manufactures guidelines on inlet / max pressure drop P 1 (inlet) P 2 (outlet) [File Name or Event] Emerson Confidential 4/22/2016 Slide 11 11

12 Spring Selection Lowest spring rate for accurate control Higher spring rate stability Spring changes gain of unit True for both self-op and pilot-op Higher gain, fast response, is more susceptible to instability 5 to 15 psig 2 to 10 psig Red Spring Orange Spring [File Name or Event] Emerson Confidential 4/22/2016 Slide 12 12

13 Pressure Sensing Internal Sensing: Pressure sensing at the regulator External Sensing: Pressure sensing remotely from the regulator to downstream piping thru control line (aka static line, sense line) Internal Sensing External Sensing [File Name or Event] Emerson Confidential 4/22/2016 Slide 13 13

14 Control Line Placement Improper control installation can cause a regulator to cycle, control the wrong pressure or respond slowly. [File Name or Event] Emerson Confidential 4/22/2016 Slide 14 14

15 Turbulent and Laminar Flow Conditions Turbulent Flow Erratic Pressure Laminar Flow Consistent Pressure [File Name or Event] Emerson Confidential 4/22/2016 Slide 15 15

16 Control Line Installation Plan the installation ahead of time using these guidelines: Make the control line as short and straight as practical Tap location critical for stability and accuracy Connect the control line 6 to 10 pipe diameters downstream of a turbulence generator (valve, tee, elbow, swage) Connect the control line to the point where the pressure is to be controlled Install control lines as you would a meter [File Name or Event] Emerson Confidential 4/22/2016 Slide 16 16

17 Control Line Tips Control Line Diameter: The control line diameter should be selected to match the connection provided on the regulator. If the control line is long (over 10 feet), the size should be increased. Control Line Length: Self / Direct - Operated Regulators: The control line can be any length. If the system is subject to quick load changes, friction in a long control line may delay the pressure wave and slow the regulator response time. Pilot-Operated Regulators: The control line can be any length. If the control line is over 10 feet long, the diameter may be increased. A long, small diameter control line can restrict pilot bleed gas flow. Sometimes just changing sensing location can improve regulator control [File Name or Event] Emerson Confidential 4/22/2016 Slide 17 17

18 Connect the Control Line at a Location Relatively Free of Turbulence A WRONG TOO CLOSE TO REGULATOR OUTLET B CORRECT CONNECTION IS 6 TO 10 PIPE DIAMETERS DOWNSTREAM OF THE REGULATOR [File Name or Event] Emerson Confidential 4/22/2016 Slide 18 18

19 Connect the Control Line at a Location Relatively Free of Turbulence A WRONG PIPE SIZE CHANGE (SWAGE) CAUSES TURBULENCE B CORRECT CONNECTION IS 6 TO 14 PIPE DIAMETERS DOWNSTREAM OF THE SWAGE [File Name or Event] Emerson Confidential 4/22/2016 Slide 19 19

20 Control Line Placement in Swage [File Name or Event] Emerson Confidential 4/22/2016 Slide 20 20

21 Connect the Control Line at a Location Relatively Free of Turbulence A B CORRECT CONNECTION IS 6 TO 10 PIPE DIAMETERS DOWNSTREAM OF THE REGULATOR WRONG TOO MUCH TURBULENCE AT AN ELBOW C CORRECT CONNECTION IS 6 TO 10 PIPE DIAMETERS DOWNSTREAM OF THE SECOND ELBOW. WITH THE CONNECTION AT THIS POINT, THE REGULATOR WILL COMPENSATE FOR PRESSURE LOSS THROUGH THE TWO ELBOWS. [File Name or Event] Emerson Confidential 4/22/2016 Slide 21 21

22 Improper Control Line Placement [File Name or Event] Emerson Confidential 4/22/2016 Slide 22 22

23 Dampen Turbulence on Direct-Operated Regulators Problem: Control line location appears to be in a good location, but regulator chatters / cycles Solution: Install needle valve so the regulator is not subject to transient pressure swings or spikes [File Name or Event] Emerson Confidential 4/22/2016 Slide 23 23

24 Control Line Valve on Pilot-Operated Regulators Avoid installing a valve in the control line / bleed line if possible A restrictive valve can cause a false signal in the control line and potentially cause cycling If a valve must be installed it should be a full port vs. a needle valve [File Name or Event] Emerson Confidential 4/22/2016 Slide 24 24

25 Needle Valve In Control/Sense Lines Needle valve installed providing 1/8 hole in sensing [File Name or Event] Emerson Confidential 4/22/2016 Slide 25 25

26 Bleed/Sense Lines Need to be Separate Avoid placing bleed and sense lines in a common location [File Name or Event] Emerson Confidential 4/22/2016 Slide 26 26

27 Spring Case Assembly Spring Case Vent Assembly, Why? It is part of the regulator system Creates Stability For internal relief [File Name or Event] Emerson Confidential 4/22/2016 Slide 27 27

28 Vent Piping Restricted vent piping can cause the following: Slow speed of response Instability Reduced IRV capacity Vent Pipe Guidelines w/irv Use same diameter pipe as vent Increase 1 pipe size for every 10 1 pipe elbow equals 3 of straight pipe 1 inch pipe Vent Pipe Guidelines wo/irv Use same diameter pipe as vent Increase 1 pipe size for every 50 1 pipe elbow equals 3 of straight pipe [File Name or Event] Emerson Confidential 4/22/2016 Slide 28 28

29 Conclusion Basic troubleshooting of any regulator will require a working knowledge of how regulators operate and how to apply them effectively To troubleshoot an application, gather as much data as possible Videos & Pictures are invaluable!! [File Name or Event] Emerson Confidential 4/22/2016 Slide 29 29