Hydrocarbon Compression

1. Check interstage line temperatures to determine which valves have been removed from a cylinder.
2. Remove disabled valves, cages, and valves in ends taken out of service and replace with gaskets. This reduces parasitic pressure loss.
3. By-pass crank-end when not in use through fuel gas lines. Saves horsepower.
4. Is engine exhaust temperature at least 600°F? Lower temperature indicates inadequate compressor utilization.’
5. Is engine “missing” more than ten times a minute? This also indicates inadequate engine utilization.
6. Can a dual acting machine operating on crank-end be changed to head-end?
7. Can a dual acting machine operating on head-end have the cylinder clearance reduced?
8. Can a dual acting machine operating on head-end be switched to dual acting without exceeding rod loading, maximum exhaust temperature or maximum horsepower?
9. Can tandem machine operating on crank-end be switched to head-end?
10. Is a tandem machine, operating on head-end, limited by maximum rod load and/or discharge temperature? If so, correct by going to tandem operation.
11. Are there any bad valves indicated by hot valve caps (suction valves can easily be identified as bad).
12. When switching to tandem, do not maximize gas production first day. Compressor will have a tendency to trip-off due to high discharge temperature.
13. When operating a compressor in tandem, the crank-end discharge temperature can be reduced at constant suction pressure and flow by closing the head-end clearance pocket and slowing down the machine. However, this is a small effect.
14. Opening a clearance pocket to reduce discharge temperature will immediately raise the discharge temperature! However, once the wellhead pressure rises due to less gas being moved, the discharge temperature will drop.
15. For wells served by a three-phase separator, adjust the threephase separator pressure down when going to tandem operation. When the compressor suction falls below 65 PSIG, liquid will carry-over from the high pressure separator and trip the compressor unless the 3-phase separator pressure is reduced.
16. About one out of three wells will start making water “hits” when the compressor suction is dropped significantly. Usually the high pressure separator will not be able to drain sufficiently fast for the first hour. It needs to be drained manually for this period. Such wells will double or triple their gas flow after making the water hits.
17. Some wells, after making water hits exhibit an increasing wellhead pressure. This may trip off the compressor due to overload.
18. Is compressor at maximum rpm?
19. Some engines bog down below rated horsepower due to inadequate fuel gas flow.
20. Check liquid dumps for leakage (i.e. dump line is cool).
21. Is compressor suction pressure not less than 20 psi below wellhead pressure?
22. Is discharge to suction bypass check valve leaking and/or blocked-in?
23. Does metered flow match the flow predicted by curve charts? About 20% of the time they do not match. Indicates bad valves in cylinders or wrong meter reading.
24. For compressor’s with meters on suction, is the engine fuel gas flow being deducted from royalty payments.
25. Is fuel gas from the suction of the compressor? On average, a compressor will use 2% — 5% of it’s production for fuel. For tandem machines operating at maximum this can be a much higher percentage.
26. Is a well soap-sticked and flowed back properly?
27. Remember that the discharge temperature from a compressor will increase as the well pressure is depleted.

Attempting to utilize a single compressor to service both the casing and tubing flows on a dual completion well can present some real problems. On one installation, both the casing and tubing were piped into the suction of the reciprocating machine. However, the operators observed that when the tubing flowed unrestricted into the compressor suction, the casing flow stopped. To “correct” this situation, a restrictive choke was placed in the tubing side of the wellhead tree. This resulted in a wellhead tubing pressure higher than the compressor discharge pressure! This odd situation resulted in a net reduction of gas flow from the well as a consequence of the compressor installation. The reason for this detrimental effect was that the wellhead compressor was too small.

field compressor to trip-off prematurely. In this case, the field operators were reporting that they could not operate a compressor suction below 70 PSIG. Their experience had taught them the following:
1. They would set the compressor to operate in the tandum mode.
2. Over a period of a few days the wellhead pressure would diminish from 120 PSIG to 70 PSIG.
3. At 70 PSIG (as indicated by the flow chart pressure recorder) the unattended compressor would trip-off.

Figure 4-3 shows that this was not quite true. The cause and solution to this problem resided in the pressure setting of the threephase, low pressure separator. As this vessel was set to hold 65 PSIG, it followed that the high pressure separator could not drain whenever it’s pressure reached 65 PSIG. The liquid level in the high pressure separator would then rise and carry-over water to the field compressor. As engine fuel was being drawn from the compressor suction line, the water overflowing from the separator entered the engine and caused it to stall. The simple solution to this problem was to reduce the three-phase separator pressure from 65 PSIG to 30 PSIG.

To further complicate the adjustment of a field compressor, one needs to be aware of certain transient effects that the well imposed on the compressor.
• Many wells, immediately after unloading liquids exhibit an increase in wellhead pressure sufficient to overload and stall the engine.
• Opening the head-end cylinder clearance valve to reduce the first-stage discharge temperature will immediately increase this discharge temperature and can trip-off the compressor. However, once the wellhead pressure rises due to less gas being moved, the head-end discharge temperature will drop.
• After switching a compressor from single-stage to tandum operation, the second-stage discharge temperature will tend to increase for a few days as the wellhead pressure drops. This often leads to compressors tripping off unless corrective action is taken.
• The immediate effects of soap-sticking a well (i.e. unloading liquids by adding a foaming agent into the well’s tubing) may be to over-load the engine due to excessive suction pressure.
• A compressor which has operated properly in a tandum mode is shut-down for maintenance and thereafter repeatedly trips off on high discharge temperature. The problem is that the well has loaded-up with liquids and the resulting low wellhead pressure is causing too high a compression ratio.

If a compressor has an excessively high second-stage (crankend) discharge temperature and a low first-stage (head-end) discharge temperature, one should proceed as follows:
• Reduce the adjustable clearance on the head-end.
• Slow the machine down.
• Balance the above two steps to restore the original wellhead pressure.

This technique switches load from the crank-end to the headend without changing gas flow. Note that to minimize horsepower the pressure ratio for both stages should be about equal. Operating with the “head-end” cylinder clearance valve wide open will tend to over-load the crank-end, under-load the head-end and waste net engine horsepower. Regardless of other circumstances, a compressor should never be run over its rated speed. However, if the machine will not come-up to its rated speed when it is runnning below its rated horsepower (as calculated above), then something is amiss with the engine.

To further complicate the adjustment of a field compressor, one needs to be aware of certain transient effects that the well imposed on the compressor.
• Many wells, immediately after unloading liquids exhibit an increase in wellhead pressure sufficient to overload and stall the engine.
• Opening the head-end cylinder clearance valve to reduce the first-stage discharge temperature will immediately increase this discharge temperature and can trip-off the compressor. However, once the wellhead pressure rises due to less gas being moved, the head-end discharge temperature will drop.
• After switching a compressor from single-stage to tandum operation, the second-stage discharge temperature will tend to increase for a few days as the wellhead pressure drops. This often leads to compressors tripping off unless corrective action is taken.
• The immediate effects of soap-sticking a well (i.e. unloading liquids by adding a foaming agent into the well’s tubing) may be to over-load the engine due to excessive suction pressure.
• A compressor which has operated properly in a tandum mode is shut-down for maintenance and thereafter repeatedly trips off on high discharge temperature. The problem is that the well has loaded-up with liquids and the resulting low wellhead pressure is causing too high a compression ratio.

As the wellhead pressure falls, the differential pressure that the field compressor must deliver increases. This is because the collection header into which the compressor discharges remains relatively constant. As this differential pressure rises, the compressor may become limited by “rod loading”. A machine may be only utilizing a fraction of the available engine horsepower and trip-out due to low suction pressure or high discharge temperature. Both of these trip points are a function of the maximum compressor rod loading which, is, in turn, a function of the differential pressure across an individual stage and the cylinder geometry. Note that at a fixed discharge pressure, a falling suction pressure always results in an increase in discharge temperature.

Naturally, operating field personnel will try to avoid repeated compressor shut-downs due to low suction pressure or high discharge temperature. The proper response would be to convert the compressor from single-stage to tandum (i.e. two-stage) operation. However, for reasons enumerated below, field personnel may choose to remain on single-stage operation and:
• If on crank-end operation, reduce rpm.
• If on head-end operation, open the cylinder clearance valve.

Both of these methods will effectively eliminate trips caused by high discharge temperature or low suction pressure. Unfortunately, they also reduce natural gas production. Why is it then, that operating field personnel do not go immediately to tandum operation to eliminate trips caused by excessive rod loading? A few of the reasons are:

• Making the conversion requires tools, valve parts and time. Also, the machine must be shut-down and re-started.
• Often, the well will produce large quantities of water or condensate for several hours after the tandum operation is initiated. The vapor-liquid separator drum on the compressor suction line may not be able to keep up with the liquid flow. Manual draining of the drum is therefore appropriate. In practice, this means that an operator must remain at the well site for half a day to monitor and control the liquid level in the compressor suction drum.
• It is human nature to avoid step-changes. Converting from single stage to tandum operation entirely alters the wells characteristics; whereas small reductions in speed or suction volume may be made gradually over a period of time.

Converting to tandum operation reduces the rod loading by spreading the differential pressure out over two stages. For a given wellhead pressure, the two-stage operation also lowers the compressor discharge temperature.

There are three fundamental limits to which all field compressors are subject:
• Compressor rod loading
• Speed
• Engine horsepower

In addition to calculating the actual engine horsepower by the above equation and comparing it to the name plate rating, the engine exhaust gas temperature should be checked. The engine manufacturer specifies a maximum exhaust temperature for the engine when running at maximum load. If this design temperature is 750°F, while the observed engine exhaust is 600°F, it is quite apparent that the engine is not running at its maximum load. On the other hand, if the cylinder clearance valve is closed a few turns, and the machine slows down (or even stalls) the engine is positively working as hard as it can. Of course, as with a car engine, adjustments to the carburetor and ignition systems can correct horsepower limits.

Do not forget that for a field compressor to develop its rated horsepower, -it must be operating at its maximum design speed. Slowing an engine down without reducing its horsepower load will raise the temperature of the exhaust gas. To economize on the available engine horsepower one can:

• Minimize pressure drop between the wellhead and the compressor suction. If the pressure difference between these two points exceeds 10 PSIG, there is an unnecessary restriction to flow. Perhaps the positive choke in the wellhead has not been removed. Oft-times the surface piping diameter has not been sized for low pressure gas. Gas heaters, necessary to prevent hydrate formation on high-pressure wells, should be by-passed when field compressors are installed.
• Withdraw gas from the suction of the compressor, rather than the discharge, for engine fuel. A 100 horsepower compressor will require 30 MSCFD of fuel or several percent of the unit’s capacity.
• Do not simply disable compressor valves when either the headend or crank-end is to be taken out of service. Remove the valve assembly completely from the cylinder. Even though the valve plate may have been removed from the suction valve, the remaining portions of the valve will still offer a substantial resistance to flow and hence absorb horsepower.
• By-pass the inter-cooler when on “crank-end” operation; alternately by-pass the after-cooler when on “head-end operation. • Wash the inter-cooler fin tubes to remove bugs and dust. Compressor horsepower required is proportional to gas inlet temperature.

The easiest, but least cost effective method, to operate a field compressor is the crank-end mode. When only the Crank-end (i.e. second stage) is in operation, capacity, compression ratio, as well as engine horsepower load and compressor rod loading are minimized. Left to their own devices, field personnel oft-times run compressors on the crank-end only. To increase the wellhead tube velocity, it is usually necessary to switch the compressor operation to the headend mode. This involves removing the crank-end cylinder valves and re-installing the head-end cylinder valves.

The head-end cylinder clearance valve should then be closed as far as possible so as to fully utilize the available engine horsepower. To calculate approximate horsepowerT the following equation may be used:

Maximizing engine horsepower and hence gas flow immediately after switching to head-end operation is helpful in achieving the tubing entrainment velocity. A gradual increase in gas flow will not be as effective in unloading the well. Therefore, the engine rpm should be set at maximum and the head-end cylinder clearance setting should be minimized as soon as the machine is put back on line.

This incident illustrates the importance of adjusting field compressor operation to maintain a minimum velocity in the production tubing. The velocity must be sufficient to entrain water, which migrates into the well, up into the high pressure separator. Based on a limited amount of data taken in gas field operation and a more substantial data base developed in the process industry, the following rule of thumb is suggested:

This equation for entrainment velocity is in the form of Stokes Law for settling of particles in a fluid. The coefficient of 1.2 will vary with gas viscosity, depth of the producing formation and the presence of surfactants in the well liquids. The reader should develop a suitable coefficient from his own experiences. Correlations developed by other workers in this field suggest that the minimum velocity to “unload” a well is greater than that shown above. Note that adding soap sticks to a well reduces the DL term in the above equation by over 50% and thus effectively lowers the entrainment
velocity.

Figure 4-1 illustrates a typical two-stage compressor. Machines of this type range from 30 to 300 horsepower. They are driven by a gas engine; fueled by natural gas. Engine speed is 250 to 450 rpm, with the compressor inter-cooler and after-cooler air fans driven by the engine. Such machines are rugged, reliable and flexible. To illustrate their flexibility, there are three principal modes of operation.

Two Stage (Tandum) Operation
Both compressor stages are fully operational. Note that the first-stage is called the “head-end” and that the second-stage is termed the “crank-end.

Head-End Operation
The compressor cylinder valves have been disabled in the crank-end (i.e. second-stage), so that only the head-end does compression work. This type of operation is summarized in Figure 4—1.

Crank-End Operation
The compressor cylinder valves have been disabled in the head-end (i.e. first-stage), so that only the crank-end does compression work.

Note that the head-end cylinder’s volumetric capacity is much greater than that of the crank-end. However, the volumetric capacity of the head-end can be adjusted with the cylinder clearance valve (see Figure 4-1), whereas the volumetric capacity of the crank-end is fixed.

In addition to these permutations, the compressor speed can be varied over a wide range, the suction flow may be throttled, engine fuel can be drawn from either the suction or discharge, and the discharge, and the discharge cooler may be by-passed.

Reducing the surface pressure by compression reduces the gas pressure in the tubing at the level of the perforations and hence increases the flow of gas from the formation through the casing perforations. The incremental flow of gas obtained from a well by surface compression is a function of many complex variables.

Gas wells that have become water-logged may double or triple production when joined to a properly sized and operated field compressor. For example, a well was producing gas at a rate of 300,000 SCFD with a compressor suction (i.e. wellhead pressure) of 400 PSIG. The compressor configuration was altered from crank-end operation to head-end operation. In effect, the volumetric capacity of the machine was doubled. Consequently, the wellhead pressure was reduced to 280 PSIG, and gas flow rose to a rate of 350,000 SFCD.

After operating for a short time in this manner, slugs of water began to pass up through the wellhead valves. The hammering sound of water entering a wellhead tree is called “water hits”. As the slugs of water raced up the tubing, the weight of water suppressing gas flow was removed (i.e. the well unloaded). Both the wellhead pressure and the flow increased. Hours later, the well performance stabilized at 780,000 SCFD and a 350 PSIG compressor suction pressure.