The field data from our compressor analysis indicated a difference in the end clearances for the head end and crank end compared to the tag and logged clearance values.  Fine tuning the clearances to the dynamic running clearance measured in the field from the analyzer will bring our simulation to be the most accurate to calculate stage flows and inter stage pressures as well as expected discharge temperatures.

Also found were inter stage pressure drops from the pulsation controls on the unit as well as the cooler.  Those pressure drops are important to note for future comparisons and for accurate inter stage pressure calculations.  Why is this important?  Knowing the expected inter stage pressures is now a sensitive set point for future comparisons for unit health and reliability assessments.  Also this will aid in formulating results for tests of other operating conditions for the unit.

With the simulation set for the correct running clearances and pressure drops, we can now input field conditions as they come in from the operations staff for health checks.   That’s right, the simulation now can be used as your first health assessment tool for any suspect problems from the field.  There are basically three alarm areas that you can now set to find a problem with your unit that would require some inspection and service.  Primarily would be flow, where the measured flow from the field doesn’t meet the simulation.  When this happens its typically also followed with the second indicator,  a discrepancy in pressures on one of the stages, if any.   Multistage units are easier to diagnose as you usually have an inter stage pressure that is much higher or lower than the simulated result and points to a problem with flow (health) of the stage.  Single stage units of course may not have a similar result.  Following that you can now narrow in on the third indicator, temperatures to find the last part of the puzzle.  Increased temperatures of any of the stage discharges could indicate a health issue needing further analysis and inspection.

Utilizing a compressor simulation program in different ways can be a very powerful troubleshooting tool when tuned to the actual field conditions.  I use simulation programs all the time to verify unit loading and health for clients that have us come out to perform compressor analysis on regular intervals.   The simulation is accurately tuned to the real field conditions and now can be used at a moments notice to quickly diagnose health with just field data of pressure and temperature input.   Being a reliability resource, you have to be able to use many tools to provide the best health assessment for your clients.

Jason Hoffman C.E.T.  from Enhanced Maintenance Solutions Inc.

Field analysis of reciprocating compressors takes some specialized equipment and techniques to ensure you are accurately reporting the machinery performance and health.  There are only a couple of products available out there such as the Windrock 6320 or the Dynalco Recip-Trap 9260.  After the equipment there is alot of training involved in utilizing the equipment properly and interpreting the results.  Contractors are becoming more common place in recent times, however not all contractors are created equal.  As with any service, do your research and select a contractor that has the experience and track record to be able to provide reliable data results.

So, now after we found a huge discrepancy with our simulation results from the unit flow meter and load from the engine/motor driver data we need to verify what we are finding.  We will assume you have selected a good contractor to provide the compressor analysis service, so we will get to analysis and data capture.  Reciprocating compressor performance testing can be a lengthy exercise, though not as detailed in instrumentation as testing centrifugal compression.  The testing conditions are key to getting accurate data and providing a good basis to make optimizing improvements to the machinery.

There are three key areas to focus on as an analyst and the first and most important is the reference marker for the compressor.  An error of 1° of crank rotation at this point, can drastically affect the calculations for unit flow and horsepower consumption.  Using a simple procedure, verifying the true phasing of the compressor will avoid any misleading results and remove any doubt on the reliability of the data.  Secondly the accuracy of data can be affected with the pressure sample or indicator valves installed in the compressor cylinders.   A full port “roddable” needle valve is the one I like the best as it can be purchased in various metal compositions for all different gasses and also can be plugged off for safety concerns.  These two areas are the most sensitive as far as the horsepower calculations.  The  third part is something we utilized in the simulation and that is the gas analysis.  This data is used to calculate the unit flow and theoretical temperatures for comparison to the field temperatures measurements.

Since the reciprocating analyzer is actually measuring the internal Pressure-Time (PT) and in turn the Pressure-Volume (PV) data for the compressor cylinder, the gas analysis is not going to affect the horsepower calculation.  This is hard to get a handle on if you have run simulations in the past as the gas analysis will affect how the simulator programs see the compressor behave and in turn change the horsepower.   As far as the analyzer is concerned all it is measuring is pressure and phase angle to produce a calculation of horsepower based on the geometry of the unit.  After that, the math from the gas analysis is included to produce flow estimates.  These are important for the evaluation of the unit health for common problems such as ring or valve leakages.

Ok, we now have field analysis data and the contractor reports a flow very close to the field flow meter and confirms that our simulation is wrong.  Now what?   Well from the data in our field analysis we have determined that we have found a huge pressure drop between stages.  That is significantly increasing discharge temperatures and now also reducing the unit flow compared to the simulation results.   Also found was a discrepancy in the clearances reported in the data for the simulation and the actual clearances determined from the analyzer data.

In Part 4 we are going to go over the fine tuning of the simulation from the field analysis data for more accurate predictions and simulations of future running conditions.

Jason Hoffman, C.E.T. at EMS Inc.

Whoa…What? Hold on a moment, how can it be that we are 150% loaded and producing more capacity than the pipeline???  We have to go through and figure out where we went wrong.  The most common errors in a simulation result from not matching the real world conditions or loading configuration.  Its usually a simple mistake that causes hours of grief in your simulation model.   Carefully check, pressures, temperatures and stage geometry to ensure all is accurate.  Much to the amazement of most, the gas analysis has some affect, but its not worth chasing down specific gas constituents to narrow in on a large discrepancy.  That is for fine tuning.

The most overlooked component in any configuration, in my opinion, is the actual cylinder end clearances that the manufacturer provides versus the actual dynamic running clearance in the field.   The tag clearances on the cylinders are based on normal factory installation conditions and don’t take into account any changes in valves or machining over the years due to wear and repairs.

Dynamic running clearance?  What is that?   Well for those that take running data on compressors with performance analyzers, we can gain the information needed to determine how much clearance the cylinder is actually compressing to.   From accurate data collected on the compressor  we interpolate the clearances from the measured Pressure-Time data.  The dynamic part of this term is that its under operating conditions and speed.

I digress, the main point here is that the clearance term in the performance equation has a large part to do with the simulations ability to accurately predict horsepower and capacity.   Incorrect pocket position measurements or valve clearances from chairs or spacers would make the simulation program far less accurate.

As we all know, garbage in = garbage out.  So if we start off with our simulation being only partially accurate, we cannot move forward to use this data for future machine optimization.   Simulation of changing conditions won’t be accurate nor will any direction to the operators for changing the load characteristics of the unit such as pocket position or stage pressure control.  Too often I see major changes happening in the field in how the unit is being operated based on incorrect data from simulations.

On the other hand…..For those more experienced in using these programs, a gross difference from your actual data in the field and the simulated data may be a sign of other problems.  These types of problems are typically better tackled by accomplished analysts with some tools on site to do the troubleshooting.

Next up in the series is a look at compressor performance from the field level and actual on machine data.  How is this different or any better than simulated data?  Why should we get accurate machinery health and performance data?  That is part 3….

Jason Hoffman @ EMS Inc.

A quick search on the web and most of the major manufacturer’s can give anyone out there the tool they need to simulate a compressor.  It’s not for the beginner,  these programs need a lot of input in the setup stages that has to be collected at field level, that is on the machine.   So doing a simulation remotely is going to require at least one visit to the machine to see how it was actually configured and what the unit actually looks like.   If you haven’t been to a site to see these machines in action, there can be some differences in design after it leaves the manufacturer that aren’t going to be on the build sheet.   Too often I see many simulations go way off course due to a simple geometry change on the machine.

FIELD TIME!!!   Get out your pens and paper and start jotting down all the information you can on the unit. Get serial numbers, cylinder bores, clearances, volume pocket positions, unloaded cylinder ends, stages, number of suction and discharge valves,  and overall position on the frame to make an accurate write-up of the units configuration.  You can never have too much information on field trips.  There is nothing worse than coming back to the office and having wished you spent another 2 minutes copying down information.    Look around the unit and jot down operation conditions like temperatures of the suction systems, discharge of each cylinder, and panel pressures.   If you can, compare any machine installed gauges to the ones on the panel.    Use your eyes and ears and feel your way around the unit to get a level of comfort with its operation.  Get a metered flow if available, that can help later on in the analysis portion.

ASK LOTS OF QUESTIONS!  The operator of the unit is the best friend you could have to get the real world experience of what goes on in a day.  They can tell you stories about pressure and speed changes, temperatures that vary with the seasons. All kinds of things that can help you determine if your simulations are going to be accurate.

ACCURATE GAS ANALYSIS.  Can’t leave without that.  You need to have an accurate account of what that thing is pushing through in a day.   Think about how the compressor is used and where a good existing sample would apply or go and have one done.

TIME TO INPUT. When you are a happy with all of that, then its time to head back to the desk and start to input what you have collected.  It’s true that the setup of the simulation run is probably 90% of the work.  After that you can start to fine tune your model to more closely fit the real world conditions.

Stay tuned for the next part of the series where we get into the simulation and then the meat and potatoes of online recip analysis.

Jason Hoffman at EMS Inc.

The balance of force and response is the key to keeping vibration levels in check on any piece of machinery, and more so with a reciprocating compressor where there are more complex forces. For vibration to change there must be a change in one of the two factors in the vibration equation.

Vibration= Dynamic Force x Dynamic Flexibility.

The key to the equation is the dynamic components of each factor. The forces and flexibility of the components change with frequency.  This is an important key to keep in your mind when vibration problems, particularly with reciprocating compression, crop up from no where.    Something has to have changed in the whole system of components and the associated forces for this vibration level to change.

From the start, getting a handle on the forces generated from the sources, be it the compression of gas or the balance of the reciprocating components will determine where the force input may have changed. Was there a change in unit operating conditions?  Was the loading sequence changed?  These factors will then guide the rest of the investigation if they are unchanged.

Next would be the flexibility of the units support structures such as mounting bases, chocks, supports, clamps or any other significant structure.   This will require some expertise in to how the unit is supported or how they are constructed to identify common failure points and areas to inspect.   The failure of a weld or crack in a support structure will adversely change the Mechanical Natural Frequency of the system.  So now components that would not be responsive to common input forces, can become amplified to the point of failure.   Simple repairs to these failed structures will improve the response to the vibration.  However, if speed is variable, dynamic force and dynamic flexibility are important to recognize as your repairs may align these resonant frequencies with other common forces.

Accurate testing of the Mechanical Natural Frequency will be paramount in determining the course of action to rectify the problem.  If the history of the machine has MNF data from previous analyses, the comparison can be made to see if the repairs to the support are making a positive impact.

The important point to remember when vibration levels change is which of the two factors have been affected. Is it the dynamic force or the dynamic flexibility?  Once you have determined this, the solution is not far away.

Jason Hoffman C.E.T at EMS Inc.

The continual battle of having hands on analysis of these assets is being “replaced” (if you can call it that) by remote monitoring programs is more rampant.  On paper, it seems like a cost savings with the ability to “analyze” your machinery at any time with an upload of some field collected pressures and temperatures. Hey, you can have a quick computer generated report come back and say all is good or “blow by” for a confirmation that something may be wrong performance wise.  On an engineering scale, brilliant.. But really is that the big money issue?  How about the risk of some of the mechanical components wearing or failing internally?  Would that be worth the cost of having an actual live analyst taking more in depth data to find and trend?  Seems to me the risk matrix was skewed with the upfront cost of getting each program running and what you are getting in return. The risk and cost of having a mechanical failure would quickly pay for a years worth of hands on analysis.

There is a place for remote monitoring and that is obvious with the application of online monitoring systems being applied to critical machinery.  The cost of these systems are more than most analyzers are and tied to one machine for life.  But taking the step past a computer generated spreadsheet for performance problems would be a vast improvement.

Forgoing vibration analysis and trending is a big item to just toss in the trash with the remote monitoring programs and the cost of one failure due to a cross head bushing or rod failure would easily pay the difference.  Vibration issues that arise with changes in conditions, loosening supports and other factors would never be realized unless an experienced operator can detect it.   Also the amount of health and reliability issues that can be concluded with actual phased pressure measurements far exceed those of the remote simulator programs.  Valve failures can be detected before the performance of the unit is affected through phased vibration analysis and defects such as spring failures or changes in valve dynamics are observed.

One of the pet peeves I have with remote monitoring services are the “optimization” suggestions that come from these expert systems.   Such as changing unit speed.  I have run into multiple clients that have gotten the suggestion that they were not operating units at top speed for increased throughput.   Fine,  but for some engine and compressor models the rated speeds were not what they are today.  Increasing the speed on these old units caused failures of the piping systems, support frames, foundations and other internal wear that chewed up the increased throughput savings in a big hurry.   Now you have physical failures to deal with.  Computer generated expert system.  How about reported pressure losses across stage coolers that is actually due to valve design or pulsation control constraints?  Again, without having an actual look at the machinery, how would you know?

For the money, having the value added service of an experienced analyst looking over the machinery, observing the data collected and making educated suggestions of performance improvements is one of the strengths we have and use at EMS Inc. The findings of one main bearing or cross head pin wear has made the program a shining star.

The major battle always comes down to when you don’t necessarily find anything wrong, for many trending sequences.  Is the program still valuable?  Isn’t confirmation of the assets health of value in the maintenance planning process?   I would think that if you can get the relationship to trust the analysis data and results, you could focus your maintenance efforts on the less reliable machinery and have comfort in knowing the “good” units are still holding strong.

If you are in a position to be selecting the remote monitoring programs, I think before you select these types of “analysis” you should consider what you are getting and what you actually are spending.   The data you get from these remote monitoring programs is limited to the the low risk-high failure rate components.  If you use a risk assessment matrix, wouldn’t you want to address the more high risk- low failure rate components?  The cost of one of these failures could easily dwarf any of the savings from the low risk-high rate components, for a long time.

Jason Hoffman, C.E.T. at EMS Inc.

In short, it takes a wide array of skills to make a good analyst.  Some analysts are former mechanics, which is great as you know the internals of most pieces of machinery.  The short fall may be in the theoretical aspect of compression and the properties of compressing gas or vibration and forcing functions.   Others may be representatives of the analyzer manufacturer, which brings many skills in the use and interpretation of the data from the instrument.   Applications may not be as varied here and the sales aspect of getting the instrument sold may steer some analysts focus.

Having an analyst that can draw from the theoretical aspect, mechanical aspect and have the exposure of working with more complex issues can round out a good analyst.   Being a troubleshooting analyst also takes a special aspect as you  have to be part detective to try and assemble data, facts, and your own interpretation of the issues to come up with the best possible solution.  This is where experience is a key factor as well as some issues are similar in fashion and the fixes found in the past may help to shape the new solution.  Its not as easy as just putting in a brace to get some vibration under control, though that is most times the fix.  MORE STEEL!   Well, having the ability to understand the Forcing Function of the vibration source, will give you better bang for the buck.  The right fix may not have anything to do with bracing at all and could be better controlled for ever, instead of just for now.   Immediate fixes are cool and visual, but having the vibration problems come back a year or so down the road with conditions change,  leaves a sour taste.

I’ve run into situations where bracing was just thrown at machinery in the hopes of “chasing the vibration” out of the building and there was really no supporting data other than “it worked before”.  Spending some time to understand the forcing function and the dynamics of the problem makes the solution more certain and less trial and error.

Performance problems are also more than just taking data and printing out the plots.  I also see a lot of “data-first” reports supplied out there and really, what are the analysts being paid for?   Does the doctor just hand you the x-rays and says “see, its a problem”?   Uh, no.. It takes more to understand issues with regard to compressor performance than just getting the data in the box.   Interpretation of the data and supplying analysis is the value in the service.  The software is getting more advanced and more “expert systems” are being developed to try and remove the subjective approach of teaching the analyst how to interpret the data.  But relying on the analyzer software has been an issue as it has removed the analyst from the decision.  I still use the 3 strike rule where its like the law, 2-3 pieces of evidence makes it more likely to be true and a confident recommendation can be made.

As you can see it takes more than just and analyzer and a computer to make a good analyst.  Knowing the difference between a simple problem and when to pull out the big guns is key to making timely and cost effective solutions.

Jason Hoffman at EMS Inc.

It seems that anyone in charge of a maintenance or reliability program these days are expecting huge savings from their programs with slashed budgets and reduced work force.   Its hard for me to understand how anyone could ask their maintenance managers to provide better control of repairs and reduced crash maintenance when they are stuck doing the same thing they have done for years.

There are many new technologies out there and Condition Monitoring programs seem to be just added cost elective services to the people with the purse strings.   But to try and trim budgets and stringing out maintenance schedules without a change in philosophy is just wringing blood from a stone.   I still see owners out there that have spent good money on their equipment and improved oil analysis programs but still rely on the minimum preventive maintenance programs printed in the manuals from the manufacturers.  Understanding the cost of NOT doing condition monitoring is the one most overlooked by many of our clients and is the hardest to put on the board room table.  But seeing the cost of one failure that could have been tracked and prevented by a regular program would make the cost less of an issue.

Adding some basic condition monitoring analysis techniques would,  at a minimum,  help to steer the maintenance priorities for these stretched out maintenance forces.    Questions about internal performance abnormalities could be identified earlier and planned maintenance is obviously less expensive than crash maintenance.  That is an irrefutable fact.

Do what you’ve always done?  Sure, it safe, no additional risk, but don’t expect to see your maintenance costs to change or your equipment to last longer. You’ll  get what you’ve always got!

Jason Hoffman at EMS Inc.

Another great article I just read at ReliabilityWeb.com with more emphasis on the front lines for an effective RCM program.  We have been helping out a client of ours in getting their reliability program off the ground and finding out how much the operations staff is not included in the program is concerning.

The cost of setting up a true condition based program inside a large Oil and Gas company is staggering.  Buying analyzers, hiring more staff, and training all compound into an upfront cost that would make most not very interested in taking the leap.  However, once that hurtle is crested, the benefits of the program are still years away with training, trending and getting the staff immersed in the program and get a feel for how its going to shape up. The risks is high with staff leaving after receiving large amounts of training, that could cripple a program in its infancy.    For our company, we have invested the time in training and experience to provide the client with the best analysis possible without all this upfront cost.

So much emphasis is placed on the RCM or CBM program that often times, the people who see and feel the machinery every day get distanced away with all the new “reliability” specialists roaming around.  Also having someone else digging up old problems, or pointing out shortcomings get the cold shoulder from the operations staff.   Not to mention that now the problems that were reported in the past and ignored, now become big money savers.   This tends to set up bunkers within the organization that will now cause the communication barriers that I talked about before.

To be a totally functional model of the RCM program philosophy, as stated in the article, the entire team of maintenance, operations and reliability have to be working together, seamlessly to get the full benefit of the RCM program.   I can not emphasize enough how important it is to involve everyone you can in the program.

I approach each of our projects with this philosophy in mind to find out who the key players are, and who is actually driving the program.  The one-man-band with his $100,000 analyzer is great, but really is just an analyst.  The next step to having a Reliability Technologist is to step outside of just the data collection window, and take a look at the entire picture.  That is where experience on our part, makes communicating to all levels more effective, and in turn, create that synergy of an effective Reliability Centered Maintenance program.

Jason Hoffman at EMS Inc.

Now the interesting part of the analysis is that from our pressure data, we find the peak internal pressures are well above what any simulator program would estimate and now your instantaneous rod loads are peaking above what the manufacturer provides as, the total rod load or combined limit.  Whats that number for?   Can we use this number to safely limit our machine to not cause a failure?

With accurate reciprocating weights, and reliable pressure data from our compressor end volumes we can calculate the maximum gas rod load and add into the mix the inertia weight of the reciprocating mass.   After we do that we can then apply the combined rod load number to our limits.  Simple right?  Should be, but without accurate weights and good pressure data its still a shot in the dark.  So in a nut shell, gas rod loads are just that, the gas forces on the rod from the act of compression.    Combined rod load or Total rod load take into account the acts of inertia to calculate the combined or total load on the rod.

Fact of the matter is, that without accurate data, you can get mislead into thinking your rod loads are in the limits or are well outside the limits . Getting real time data from the unit, in this case, provided much insight into the actual rod loading being exerted on the machine.  Something that the simulation program or a simple spreadsheet couldn’t estimate.

Jason Hoffman at EMS Inc.