accuracy of tested r ratios - alrdc - homealrdc.org/workshops/2017_2017gasliftworkshop... ·...

Oct 23-27, 20172017 Gas-Lift Workshop 1

Accuracy of Tested R Ratios

Ken Decker

Gas-Lift WorkshopHouston, Texas, USAOctober 26, 2017

Introduction

The current gas-lift shop test procedure requires the technician to adjust the opening pressure (PvoT)…

BUT

…he is NOT adjusting the open pressure. He is actually adjusting the bellows pressure (Pbt) and trusting the published R ratio is correct.

When the technician prepares the valve and tests only the PvoT, he has no way of knowing if the valve’s actual R ratio is the same as the published R ratio.

As a result

For any given PvoT, the bellows pressure could be less than anticipated if the R ratio is larger than the published amount.


Published R ratios

• The published R ratio for a square-edge MONEL™ port assumes the valve seals at a diameter 0.006 inch larger than the port diameter regardless of bellows pressure or port material strength or width of the lap band or bellows size.

• Measurement of the outer seal diameter of ports from VPC™ tested valves has shown the seal diameter is larger than published.

• Test data from AVT and MVT test devices have shown the tested R ratio is consistently larger than published.

• Additional tests were conducted at LSU and found both the seal diameter and R ratio to be larger than published.


R ratio Test Method


• When the PvcT is tested along with the PvoT, it is possible to calculate the

valve’s true R ratio. This method of determining the R ratio is free of

any assumptions concerning bellows effective area or valve sealing

area.

• The formula specified by API 19G2 in J.1.2.3b to calculate Rtef from test data is…Rtef = (PvoT – PvcT) / PvcT

• The formula used to calculate the R ratio is…R = (PvoT – PvcT) / PvoT

Close Pressure Testing?

• Close pressure testing and calculating the valve’s true R ratio is about cost. The ability to perform the tests has been demonstrated for many years.

• Test equipment is much better than before and now we have D/A systems and computers to record the test.

• But there is another reason we don’t test close pressure – status quo. “We’ve always done it this way and we haven’t had any problems.”

• We have had problems – not many but when we do have problems we accept it as part of gas-lift. We don’t do a good job of analyzing the cause of failure.

• Some of those problems may have been due to R ratio, stem travel, or loadrate. If close pressure testing is conducted, we have a better chance at diagnosing the problems.


Why haven’t we tested close pressure?

• It has been suggested by a gas-lift valve manufacturer that “the accuracy of the open and close pressure test is insufficient to calculate an R ratio”. Considering the current test equipment and methods, this is a valid statement.

• When using IPO valves, a close pressure test was considered unnecessary. The backpressure regulator theory supported this conclusion.

• The cost of the close pressure test equipment, training, expertise, and the time associated with performing a close pressure test is borne by the gas lift valve manufacturers.

• When only an open pressure test is conducted, every valve passes –regardless of the R ratio, stem travel, or loadrate. No decisions have to be made about accept, reject, or repair.


The Need

• When gas lift is practiced at annulus operating pressures less than 1200 psig, the small differences between published and tested R ratios have little effect. The industry now has IPO valves that are capable of being set at pressures well above 2000 psig.

• The small errors that were acceptable at pressures less than 1200 psig are NOT small errors at pressures above 1200 psig. The errors will consume the gas-lift design safety factors. To confidently practice gas lift in high pressure applications, close pressure testing is essential.

• The ability to perform troubleshooting analysis is much improved when the valve’s tested properties are available.

• Valve performance models are based on PvcT – not PvoT. If you use a published R ratio to calculate PvcT, the performance model will NOT be accurate.


The Need

• For IPO valves, one of the main design concepts is the annulus pressure drop at each lower valve. The amount varies depending on valve size, model, expected production rate, and personal preference. The bellows and close pressure (Pbt and PvcT) are lower than anticipated when the R ratio is larger than published.

• Therefore, the annulus pressure will have to drop an amount equal to the difference between the bellows pressure calculated with published R ratios and the actual bellows pressure of the valve in order to close the valve at the design production transfer pressure.

• The design practice of pressure drops between valves was actually compensating for a larger R ratio than published. The amount was arbitrary depending on how large the actual R ratio was. Sometimes 10 psi worked and sometimes you needed 20 psi.


Required R ratio Accuracy

• The difference between the tested R ratio and the published R ratio will be referred to as ∆R. In most cases, ∆R will be positive.

• The difference between the bellows/close pressure using the published R ratio and the bellows/close pressure using the tested R ratio will be referred to as ∆Pbt. In most cases, ∆Pbt will be negative.

• Since a larger R ratio than published causes the bellows pressure to be lower, the annulus pressure will have to drop an amount equal to ∆Pbt

in order to close the valve.

• Therefore…




The table serves as a reference to decide if the tested R ratio is acceptable for service. It can be applied to any valve or port size.

To use the table, find the difference between the tested R ratio and the published R ratio (∆R). For the PvoT of the valve, scan to the right until the table value is greater than ∆R. Read the ∆Pbt amount in the column heading. That is the amount the bellows pressure will be lower than calculated using the published R ratio.

The question of R ratio accuracy becomes, “How much bellows/close pressure tolerance (∆Pbt) is allowable?”

Of course, if the tested R ratio is used in the design, the table isn’t needed.



You can estimate the required pressure measurement accuracy by multiplying the table amount by 1000.

This accuracy includes not only pressure measurement but also temperature, test method, and analysis of test data.

For example, if your measurement system has an accuracy of +/-5 psig, the temperature could be +/-1 degree, and you are reading from an analog gauge (+/-3 psig) the total error is 10 psig.At PvoT of 1000 psig – best is ∆Pbt 10 psiAt PvoT of 1200 psig – best is ∆Pbt 12 psiAt PvoT of 1500 psig – best is ∆Pbt 15 psi

The higher the PvoT, the better the test system has to be or use larger pressure drops at each valve.

“What are the implications of ∆R on the test equipment?”

PERTT Test Lab Equipment

• A series of tests were funded by Oxy and conducted at Louisiana State University at the PERTT lab with help and assistance from LSU staff and graduate students to determine if the R ratio could be tested with sufficient accuracy.

• The PERTT lab test system has the ability to…

– Control upstream and downstream pressure independently

– Measure pressure to 0.05% accuracy and 0.03% repeatability at 2500 psi with 5 point NIST calibration. At full scale +/-1.25 psi

– Recording at 10 samples/sec

– Measure stem travel during test to +/-0.0003 inch

– Measure bellows housing temperature to +/-0.3 degree F

– Import test data to spreadsheet program, graph, and analyze


PERTT Lab Test Equipment


Test Program

• The instrumented valve was tested at three bellows pressures (Pbt); 1040, 1215, and 1478 psig. At each bellows pressure, a new square-edge MONEL 400 port was installed in the valve prior to charging the valve. The valve and port were NOT lapped together.

• The valve was charged to pressure while measuring/recording the bellows pressure. The charging fixture was removed, the valve chilled to 60F, and the valve placed in the test fixture for open and close pressure testing.

• The open and close pressure tests were performed three times in succession while recording the upstream and downstream pressure, stem travel, dome pressure, nitrogen temperature and bellows housing temperature.


Measuring the Seal Area


At the completion of the tests, the bellows pressure was relieved and the port removed from the valve. The port seal diameter was measured with calipers and the help of a 14X loupe (+/-0.001 inch).

Port Seal Diameters

PERTT Lab Test Results

Bellows Pressure Seal Diameter Seal Area Measured R ratio ∆Pbt

1040 0.205 0.0330 0.04286 5

1215 0.205 0.0330 0.04286 ~6

1478 0.204 0.0327 0.04244 ~7

• MONEL port material tested at 91 ksi compressive yield strength (0.2% offset). This is on the very high side of material strength. Normally MONEL cold rolled has a compressive yield strength of 45-80 ksi.

• The seal diameters should increase with increasing Pbt – but they don’t.

• When the inside diameter was measured it was found material was being displaced to the inside diameter of the port. The seal area did increase with increasing Pbt .

• MONEL work hardens when deformed. This will increase the material strength more than the tested strength.


Result of Open/Close Tests


The test data was imported into a spreadsheet program and graphed as pressure versus time. A range of pressure was selected for open and close pressure. The range was analyzed using a Monte Carlo method to arrive at the final R ratio.

Calculated R ratio


The above tests were performed without lapping the valve to the port. During the tests, a leak was detected at the valve/port interface. The leak was minor but in normal circumstances would not have been tolerated. The valve would have been lapped to the seat. This would have caused an even larger R ratio.

In summary, the tests indicate the open and close pressure can be measured consistently and with sufficient accuracy to compute an R ratio even when the valve is leaking.

Sources of Measurement Error

• The open and close pressure are measured using the same transducer or gage. The accuracy of the R ratio is a function of the repeatability of the gage – not the accuracy.

– Piezoelectric transducers are incredibly repeatable – usually an order of magnitude better repeatability than accuracy.

– Analog gages have hysteresis, the bourdon tube fatigues, the linkage wears, and frequent calibration is required. An analog gage with 0.25% accuracy does NOT have 0.025% repeatability.

• If an analog pressure gage is used, human error occurs due to parallax, eyesight, and judgement. If a digital gage is used, there is very little room to misread the pressure.

• Neither the open nor close pressure is a stable pressure – it changes. This leaves room for interpretation. For an accurate open or close pressure, the data should be recorded and a statistical analysis method should be used to determine the open and close pressure.


Sources of Measurement Error

• There are errors associated with the D/A system.

– A 16 bit ADC can resolve only 5 significant digits

– The transducer and D/A system are temperature sensitive

– EMI Noise in the signal adds a layer of doubt

– If the recording speed is too slow, you will have to interpolate

• Temperature is not normally measured during the tests

– At 1200 psi bellows pressure, a 1 degree change will cause a ∆Pbt of 2.7 psi

– Handling the valve for 15 seconds will change the temperature 4.4 degrees.

– The act of opening the valve to full stem travel increases the nitrogen temperature but as the valve closes, the temperature drops back to original if the speed of open and closing are the same.

• The test method can affect the recorded pressure

– Open and close pressure test must be at the same speed

– Rapidly ‘working’ the valve will increase the bellows temperature

– Leaks in the system cause doubt


Summary

• I do not believe ∆ Pbt of less than 10 psi can be achieved beyond a PvoT of 1200 psig using the current test method i.e.; analog pressure gauges, a technician’s quick eyes, a clipboard and pencil, and good judgement.

• ALL of the close pressure tests conducted to date have shown the R ratio to be greater than published – regardless of valve size or port size.

BUT

• If the annulus pressure is below 1200 psi and the tested R ratio is less than 20% greater than the published R ratio – you are good to go.

AND

• Keep that heavy handed technician away from the lapping compound. This is especially true with tungsten carbide seats.


Summary

• Using a test system similar to the PERTT test lab equipment…

– It is possible to test PvoT and PvcT with sufficient accuracy and consistency to enable R ratio calculation at PvoT pressures up to 2500 psig and ∆Pbt of less than 15 psi.

• Bellows housing temperature should be measured while performing open and close pressure tests.

• Both opening and closing tests must be conducted SLOWLY.

• For valves set at PvoT above 1200 psi we should be testing PvcT.


Summary

Scrupulous attention to detail is required during the gas-lift design phase.

Why would you expect to prepare the valve for service in the field with any less attention?

Test the R ratio


Acknowledgement

• Thank you to Occidental Petroleum for funding the tests.

• Thank you to Louisiana State University for allowing access to

the PERTT lab for the tests.

• Thank you to Paulo Waltrich (VPC™ Administrator) and Wesley Williams (PERTT lab Manager) for advice and assistance.

• Thank you to Renato Coutinho and Khadhr Altarabulsi for

assisting with the test program.

• The opinions and conclusions expressed in this presentation are Ken Deckers and not necessarily those of Occidental Petroleum, LSU, Paulo Waltrich, Wesley Williams, Renato Coutinho, or Khadhr Altarabulsi


IF YOU HAVE QUESTIONS, I WILL TAKE THEM AT THIS TIME.

For more information about the effect of R ratios on IPO valvesvisit…www.onepetro.org/journal-paper/SPE

SPE-186110-PA“Effect of R Ratio on Performance of Injection-Pressure-Operated Gas-

lift Valves”

SPE-189450-PA“Gas Lift Valve R Ratios”

Thank you for your attention.



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accuracy of tested r ratios - alrdc - homealrdc.org/workshops/2017_2017gasliftworkshop... ·...

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