Reciprocating compressors in oil & gas applications are subject to stringent operating conditions that may lead to accelerated wear on the pressure seals. It also applies to combustion engines.
Compressor performance maps are a graphic model of the thermodynamic behaviour of the machine, usually in “as new” condition, and methodologies to graphically account for wear in such maps are not abundant in the energy industry.
Some operators of recip compressors in remote areas or areas with little support and service availability can have difficulties in finding the correct setting for the variable volume clearance pockets (VVCP) in a performance map because both engine and compressor have accumulated a significant amount of wear. This is particularly true in presence of network pressure fluctuations.
Field analysts of reciprocating compressors may have difficulties in matching the gas flow obtained from the performance map with the one obtained with an electronic analyzer or a meter. It also applies to field people with nothing but a performance map at hand.
Several reciprocating compressor map formats are available in the energy industry, some more practical than others. Here we present useful formats that can be of interest for those linked with the field analysis or this type of machinery.
Another idea behind this paper is to let the reader to evaluate the suitability of the map formats presented here, and provide a feedback if desired. Self explaining graphs are presented in order to reduce this narrative to a minimum. Verbal clarifications will be provided to the audience.
Triple performance map for recip compressors
Having power and flow curves together with variable volume clearance pockets (VVCP) setting as a function of suction pressure is a convenient way to help operators to find out the recommended setting for the VVCP. See Figs. 1 and 2.
Figs. 1 and 2 are the so designated Basic Performance Maps (BPM) or match available power map in as “new condition”. The key points for the three curves are the vertices where the curves break. Of course the three curves break at the same Ps where the driver becomes overloaded.
Operation outside performance map. Approximate method
Figs. 3 and 4 help operator to estimate in the field the performance for an arbitrary VVCP setting.
Let´s suppose operator sets the VVPO in the S (sensitive) setting shown in Fig 3. Resultant flow can be estimated by linear extrapolation base upon the doted flow line extending from the vertice. X, Y and X´ can be determined from the vertical axes scales or with a measuring tape on the graph. Y´ flow change can be cleared from the proportionality relationship:
Use same approach for power. Such a way one can skip the use of a PC on the field.
A sensitive (S) setting can be either a result of an incorrect adjustment from the operator or a result from a pressure fluctuation.
Fig 4 illustrates how to deal with S settings. The slope of the brown power line, result of suction pressure fluctuations, is in between the slopes of the lines at vertices. At its own, slope of the flow brown line, is fairly parallel to the lines at vertices.
Engine wear map
Fig 5 shows the Basic Performance Map (BPM) with wear only in the engine side. The wear in the compressor is kept at 0% in this analysis. The blue lines represent the performance with the engine in “As new – 0% wear” condition, whereas the green lines represent the performance with the engine in the “worn out – 100% wear” condition.
Wear level on the engine side can be conveniently linked to the measured cold compression (or with elapsed time) in power cylinders. As cold compression drops (EG from 240# to 150#), the HP and Q curves drop from the blue lines to the green ones. At its own, VVCP setting line rises from the blue line to the green one. Focus on the vertices of the performance curves.
Incidentally, the behaviour of the VVPO curves is acting as a “pivoting stick”, with the pivot located somewhere down the Y axis.
By having the blue and green lines, corresponding to 0% and 100% respectively, one can figure out the current VVCP line for any intermediate wear level by just drawing the corresponding VVPO line (let’s say 80% wear) in the appropriate angular location of the “pivoting stick”. See black line in the bottom (VVCP) part of Fig. 5.
Next, flow and power curves can be easily located by extending a vertical line from the intersection of the “pivoting stick” with the X axis. See red line in Fig. 5 for 80% engine wear alone. Red circles show the location of vertices. Black lines represent the resultant BPM with 80% wear in the engine side.
Field operators can quickly update a BPM without any software by knowing/guessing the amount of wear in the engine side. Use this setup also for fuel/ambient federation.
Compressor wear map
Fig 6 shows the BPM with wear only in the compressor side. The blue lines represent compressor performance in “as new” condition, whereas the red lines represent performance in “worn out – 100% wear” condition. Wear levels can be conveniently linked to the measured or current “Lumped Volumetric Efficiency (VE)” of the compressor. By comparing current Lumped VE against as new one one can elaborate a % wear indicator for compressor cylinders. Such indicator needs further elaboration not provided in this paper, but certainly can be handled with the help of an electronic analyzer.
For the purpose of this proposal, a 50 points drop in the “As New” SVE is considered 100% wear. Two industry accepted thresholds for low SVE do exists: one is 30% (alarm), when thermodynamic formulae starts loosing accuracy due to the amount of hot gas trapped (and ready to mix with cold gas at the end of re-expansion event) at the end of discharge event, and the another one is 15% (trip) when flow reversal plays a major role in the discharge valve plate dynamic, shortening its life by means of high impact velocities against the seat. Theses values should be adjusted in a case by case basis, taking into account the gas handled and the maximum VE attainable by a given cylinder design.
Lumped VE can be derived from the electronic analyzer readings and arithmetically elaborated depending on the number of stages and cylinders. In a multistage compressor the key VE is the one associated to the first stage. In a multicylinder stage one can use a (weighed) average.
In Fig. 6 100% means the Lumped VE dropped 50 points from the As New value whatever it is. The performance curves move from the blue lines to the red ones. Same “pivoting stick” effect as stated in 4 applies here for the three curves.
The green line in Fig 6 together with the green circles depicts the location of the vertices in order to sketch an arbitrary 80% wear curves for this map. Black lines depict the resultant BPM with 80% wear only in the compressor side. 80% wear represents a drop of 40 points (80 x 50/100) in the AS New VE whatever it is.
Equal wear map
Fig 7 shows the BPM with equal wear for both engine and compressor sides. The blue lines depict the location of the vertices for both the 100% wear engine and 100% wear compressor. The vertices for the HP and Q curves drop vertically in location along the red line, whereas VVPO curve stays always on the “as new” blue line.
For adjusting both HP and Q curves for a given common wear level (let’s say 80%), one drops proportionally the vertices down along the vertical red line. Red circles show the approximate position of the vertices for the HP and Q curves having 80% wear in the engine and 80% wear in the compressor. Again, the VVPO curve is the blue line. Black lines show the resultant BPM.
Performance map with combined wear
Fig 8 shows the BPM with Combined Wear (Wear Map) with the full spectrum of the effect of combined wear on both engine and compressor sides. Blue lines show the BPM with both engine and compressor in “as new – 0% wear” condition and also in “worn out – 100% wear” condition. Green lines show the BPM with 100% wear only in the engine side, whereas red lines show the BPM with 100% wear only in the compressor side.
The objective of this map is to derate the BPM for an illustrative engine-compressor set which reports 50% wear on the engine side and 25% wear on the compressor. Such a case have a differential wear of 50% – 25% = 25% toward the engine side (engine has 25% more wear than compressor).
Firstly, one draws the vertical line denote as “A”, and drop both the HP and Q curves half way (50%) along A. Secondly, one draws lines C and D parallel to original HP and Q curves. Next, one draws another vertical line denoted as “B” starting in a point in the X axis corresponding to 25% differential wear on the engine side.
One finds the derated HP and Q vertices on the intersections between lines B, C and D. Derated vertice for VVPO curve locates on the intersection between B and X axis. One finds the approx. slope for the VVPO curve following the grey lines spectrum.
Final derated BPM is comprised of the black lines.
Constant volumetric efficiency map for recip compressors
Fig 9 and 10 show head end constant suction volumetric efficiency (SVE) lines C1 HE in “as new” condition on the bottom portion of the BPM for a four stage recip compressor. Nominal values are:
- Ps = 60#
- Pd = 1350#
- VVCP setting = 3.8”
- HE SVE = 29% (interpolated value – Approx.).
By opening VVP to 5.2” SVE yields 18%.
Fig 10 shows a more complete picture with VE lines for different discharge pressures: From Nominal values, if Pd increases to 1450# then the 18% SVE line drops slightly, thus increasing the SVE to 29.25% (interpolate value – Approx.).
A characteristic map for crank end constant suction volumetric effciency (DVE) is shown in Fig 11. This cylinder has a VVCP in the head end side, which setting lines are also shown. DVE changes with VVCP setting because interstage pressures change with it. Nominal values are:
- Ps = 60#
- Pd = 800#
- VVCP setting = 4.6”
- CE SVE= 81.9%
If discharge pressure decreases to 700#, then CE SVE decreases to 79.5% (interpolate value). In the event suction pressure increases to 65#, then CE SVE decreases further to 77.8% (interpolate value).
It is apparent from Fig 11 that volumetric e ciency maps, as shown, have a good accuracy being capable to estimate one decimal value. But now, what if this cylinder has some wear in it?. How can one determine the % wear from an electronic analyzer reading?.
Measuring wear with electronic analizers
Electronic analyzers provide volumetric e ciency readings, which can help in determining current wear in the compressor (and engine) side. As proposed in Chapter 5 above, % wear can be estimated once both current and as new VEs are at hand.
Figure 12 shows a multistage compressor (same depicted in Figs 9 and 10) with a VVCP setting of 3” and a “current” SVE of 40%, as measured by an electronic analyzer. Theoretical “as new” value for SVE is 41% according to constant VE lines. % Wear can then be deducted arithmetically as follows:
- VE Drop (Wear):
- Current wear: 100(41 – 40)/50 = 2%
- Alarm: 100(41 – 30)/50 = 22%
- Trip: 100(41 – 15)/50 = 52%
Where 50 is the maximum SVE allowable drop, 30 is alarm level and 15 is trip level as proposed in Chapter 5.
The above analysis is valid only for theoretical SVEs greater than 30%. In the event theoretical SVE is less than 30%, then the cylinder is poorly designed, and alarm and trip levels do not apply (would be negative). In order to estimate the lumped current VE for characterizing wear in a given cylinder, a set of four values should be available, namely:
• HE SVE.
• HE DVE.
• CE SVE.
• CE DVE.
However DVE should be used with caution, because discharge valve malfunctionings causing flow reversal can virtually and erroneously increase DVE readings taken with an electronic analyser.
Mathematical manipulation of such parameters in order to abtain a lumped value is a challenge out of the scope of this paper.
Updating performance maps according to accumulated wear can help field operators and analysts to estimate the recommended VVCP settings due to changing process conditions (Ps, Pd) and to match flow readings either from electronic analyzers or meters.
Electronic annalizers commonly provide theoretical VEs, same as VE maps in Figs 9 through 12, but VE maps provide the analyst with a more complete picture of the cylinder characteristic behavior in a wide operating range.
Modern recip compressor modellers can be of paramount help in order to produce the VE maps required to estimate % wear in the cylinder.
Further refinement of the algorithm suggested in this paper can produce useful tools to account for wear in the recip compressor performance maps.
Autor: Luis Infante