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Rotating Equipment
power consumption due to the higher compression ratio and discharge temperature.
Unloaders on suction valves. This method is the most
widely used to control the load on double-acting reciprocating compressors (FIG. 3). Load control is achieved by loading or un- loading the suction valves with pneumatic actuators, which are usually operated manually, depending on the required capacity. The unloaded valves are open during the compression cycle, so that the gas moves in and out of the compression chamber through the suction valve. This flow is not sent downstream, thus reducing the capacity of the machine and energy consump- tion proportionally to the amount of gas that is not compressed. This system does not permit a fine control of the load, and the quantity of load steps that can be achieved depends on the
number of cylinders and compression stages, along with the number of valves installed on each cylinder end. Typically, load stages with which the machine is designed are 0%–25%–50%– 75%–100%. As these are fixed steps of load, a certain quantity of excess gas should be recirculated to the compressor suction to deliver only the required flow to the process. The efficiency of this method is good because the adiabatic power is reduced proportionally to the flow reduction. However, the gas recir- culation through the open valves leads to some power losses, which may be important.
Variable clearance pocket. As pointed out earlier, the clear-
ance volume present in the cylinders of a reciprocating com- pressor will change the volumetric efficiency. Any change in the clearance volume of the cylinder will affect the maximum load capacity of the compressor.
Previously, compressors were designed with “clearance pockets” that provided one or two additional steps of loading on each cylinder, depending on its location (crank end, head end or both). Later, with the development of electronics, some manufacturers have designed hydraulic control systems that en- able automatic clearance volume with continuous fine adjust- ments during the compression cycle (stepless-capacity control systems) at rates from 50% to 100%. FIG. 4 shows a manually actuated valve that allows one additional load step in the head end of the cylinder, along with another valve that is hydrauli- cally actuated to permit stepless-capacity control.
FIG. 5 shows a new control system in which the hydraulic unit is not needed because the capacity control is achieved due to a reference gas, typically the process gas compressed by the ma- chine. The clearance pocket can be mounted in both cylinder ends, and it is possible to achieve a regulation range from 70% to 100%. This system works with a standard valve, and is easy to install and maintain.
FIG. 6. HYDROCOM control system.6 Source: HOERBIGER.
C BDC TDC Cr B A P D Dr V C BDC TDC Cr B A P D Dr V Energy savings compared to recycle valve control
FIG. 7. Capacity regulation reverse flow.6 Source: HOERBIGER.
Gap Piston Cylinder Piston ring A – A A C A a ba b
FIG. 8. Gas recirculation areas through piston rings.8
FIG. 4. Clearance pocket valves, handwheel operated and hydraulic. Source: Dresser-Rand.
Pocket control valve
}
Compressor cylinder Fixed volume pocket
Control gas connection
FIG. 5. Clearance pocket—gas-controlled stepless pocket (GSP). Source: Dresser-Rand.
Rotating Equipment
83
Reverse flow capacity control systems. Capacity-control
systems based on a reverse flow effect present an interesting evolution. While using the traditional unloaders on the cylin- der effect where the valve works is unloaded during the whole cycle, the reverse-flow systems allow the valve to work only during a part of the compression cycle, thus obtaining a step- less-capacity-control system. This system was developed in the 1990s; it has evolved and is considered very reliable (FIGS. 6 and 7). Several manufacturers have developed their own ca- pacity-regulation system based on the same principle. They all offer good energy savings because the capacity control ranges from 20% to 100% capacity.
MODERN DESIGNS OF COMPRESSOR COMPONENTS
Other improvements in compressor design include:
Rider bands and piston rings. From FIG. 1, between 2% to 7% of the energy used in a reciprocating compressor is due to fric- tion losses, and 0.5% to 1% of energy loss is attributed to gas re- circulation through piston rings and packings. The losses derived from internal recirculations have an influence on the volumetric efficiency (between 0.5% and 3%), thus affecting the flow capac- ity of the cylinder.7 Using quality rider bands and piston rings not only influences reliability, but also affects total energy consump- tion and the pumping capacity of the compressor.
Great advances in plastic materials for the manufacturing of rider bands and piston rings have increased the service life of these elements, thus reducing wear and maintenance costs due to the high-wear. Likewise, internal recirculation through the piston elements are minimized due to the limited wear, even the non- lubricated, which tend to wear faster. Rider band and piston ring materials are usually of a combination of PTFE with graphite or other elements, depending on process gas conditions.
New piston designs allow installing the rider bands with an appropriate distribution, which minimizes losses and friction between the liner and piston rings (FIG. 9). For example, dis- tributing the sealing segments in the central part of the piston will lead to a higher service life, because the elements work at a lower differential pressure, which is distributed across the pis- ton rings. Internal recirculations are minimized due to new de- sign. Using this configuration, rider bands are grooved to keep them from acting as seal elements. Sealing should be done by the piston rings. The efficiency of the sealing elements of the piston is strongly influenced by gas cleanliness and good cylin- der lubrication (on machines where it is required).
Valves. Both suction and discharge valves have traditionally
been a huge reliability problem for reciprocating compressors. One of the most renowned field studies suggests that 36% of faults due to unscheduled reciprocating compressor shutdowns are linked to valve problems.9 The study was done in 1996, and valve design has evolved. New data shows that a longer service life is now possible with improved valve design and new con- struction materials.
When a reasonable lifetime of compressor valves is ob- tained, manufacturers’ efforts are directed to reducing losses by friction. Valve manufacturers have achieved performance improvements with energy consumption savings up to 2% over conventional valves.
Simulation tools, such as computational fluid dynamics (CFD), have enabled redesigning conventional valves to ob- tain substantial improvements in energy efficiency (FIG. 10). It is possible to reduce the gas velocity passing through the valve and the formation of vortexes; both have a great impact on the efficiency of valve, which can be calculated based on the effec- tive flow area (EFA).10 It refers to the throat area for an ideal discharge nozzle that can be calculated (for non-viscous in sub- critical flow) from the ratio, Ks:
Ks m % GFA P01 RT2 01
1PS2 P01 2 PS2 P01 1
FIG. 9. Piston rings installed in a central position reduce internal recirculations, as the differential pressure between them is lower than in other designs. Source: GE Oil & Gas.
84MAY 2015 | HydrocarbonProcessing.com
Rotating Equipment
Once Ks is calculated, the valve efficiency can be calculated: EFA = GFA × Ks with: GFA = OP × Lift
where:
Ks = Flow coefficient
Lift = Stroke from the closed to the open position
ṁ = Measured mass flowrate
P01 = Total pressure upstream of the valve
Ps2 = Static pressure downstream
T = Total temperature upstream γ = Ratio of heat capacities Δp = Pressure drop across the valve Φ = Valve diameter
EFA = Effective flow area GFA = Geometric flow area OP = Opening periphery
ID = Inner diameter OD = External diameter.
The efficiency of the valve increases by increasing the EFA parameter, so it can be deduced from the listed equations that valve efficiency will improve by increasing within certain limits one or more of these parameters:
Opening periphery (OP) is the length of the perimeter of
the gas passage on the seat. It can be calculated for a ring valve as the sum of the OD and ID of all rings inside the valve.
Lift is the maximum stroke from the closing to opening posi-
tions of the rings or plate inside the valve.
Ks is the ratio between the ideal nozzle throat area (EFA)
and the GFA.
Packings. About 3% to 10% of the energy used by reciprocat- ing compressors is generated by friction losses in the packings. Another 0.5% to 1% of energy loss is due to leaks in the pack- ings and piston rings. The packings also have a high influence on reliability, as they are subjected to significant wear. Preventing leaks to the environment and flares has been addressed by new designs developed by OEMs in energy efficiency and reliability.
Packing does influence energy consumption for compressors. It is estimated that each standard ring set tangential/radial may represent an average energy consumption of about 5 kW. This power value multiplied by all the rings installed on the machine is not negligible. Manufacturers have developed new designs to minimize friction between rings and the piston rod. A new design (FIG. 11) uses a single-ring design with a pressure balancing groove that reduces friction losses up to 40% over standard packings.a
Packings are responsible for most gas leaks, and have environ- mental and economic impacts (FIG. 12). To dilute and minimize leakage of process gas, low-emission packings have been installed
FIG. 13. Packing with nitrogen as buffer gas.11 Source: HOERBIGER.
pgas
poil > pgas
poil pressurized oil Specially designed oil seals
FIG. 14. XPerSeal packing.12 Source: HOERBIGER.
0.00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average leakage Average leakage Service life BCD ring RT ring
(Set of one radial cut ring and one tangential cut ring)
Packing leakage erratic and elevated due to changes in cup pressure distribution
FIG. 12. Comparison of average leakage BCD ring vs. radial/tangent ring design.11 Source: HOERBIGER.
Energy savings BCD ring Conventional tangential/radial ring
Supply pressure, bar 0 10 20 30 40 50 60 70 1 2 3 4 5 6 7 Loss b y friction, kW Ener gy loss, %
FIG. 11. a) Power savings on BCD packings; (b) BCD ring. Source: HOERBIGER.
FIG. 10. Simulation CFD of a valve. Typical discharge valve arrangement.10 Source: GE Oil & Gas.
85
Rotating Equipment
equipped with inert gas injection (usually nitrogen). This design is well-proven and validated by the successive revisions of the API 618 about reciprocating compressors. FIG. 13 shows a pack- ing system equipped with nitrogen injection.
Another advanced design aims to achieve zero leakage and minimize friction losses, as shown in FIG. 14.b In this case, pres- surized oil is injected into the packing at a pressure slightly higher than the gas pressure, so the leak is avoided. Oil consumption is minimal, and, due to oil flow friction losses, the temperatures reached in piston rod and packing rings are lower than those that are obtained in other designs.
CONCLUSIONS
There is an important improvement margin in terms of energy efficiency related to reciprocating compressors. Some modifica- tions and best practices can be implemented easily and with very little investment. Some improvements can be obtained simply by adjusting the loading of the compressor to the real needs of the process, thus avoiding unwanted gas recirculation. The daily work of checking process conditions and machine capacity is crit- ical to improving compressor efficiency and performance. The reliability engineer must be aware of new developments to take advantage of solutions and practices that are efficient in terms of energy and to minimize gas leakages and recirculations.
NOTES
a A new design with reduced friction is the Balanced Cup Design (BCD) made by
HOERBIGER.
b XPerSeal, developed by HOERBIGER.
LITERATURE CITED
1 Dimoplon, W., “What Process Engineers Need to Know About Compressors,”
Compressor Handbook for the Hydrocarbon Industries, Gulf Publishing Company,
1979.
2 Vila Forteza, M., “Eficiencia energética y actualización tecnológica de com-
presores centrífugos en la actual coyuntura económica, Revista Mantenimiento, No. 267, September 2013.
3 Bloch, H. P. and J. J. Hoefner, Reciprocating Compressors Operation and
Maintenance of Reciprocating Compressors, Gulf Publishing Company, 1996.
4 EFRC Website, http://www.recip.org/173.0.html.
5 Faulkner, H. B., “An investigation of instantaneous heat transfer during compres-
sion and expansion in reciprocating gas handling equipment,” Massachussets Institute of Technology, 1983.
6 Stachel, K. and M. Wenisch, “Improved control concepts for reciprocating com-
pressors in refining processes,” ERTC 18th Annual Meeting, November 2013, Budapest, Hungary.
7 Hanlon, P. C., Ed., Compressor Handbook, McGraw Hill, 2001.
8 Liu, Y. and Y. Yongzhang, “Prediction for the Sealing Characteristics of Piston
Rings of a Reciprocating Compressor,” International Compressor Engineering Conference, 1986.
9 Leonard, S. M., “Increasing the Reliability of Reciprocating Compressors on
Hydrogen Services,” NPRA Maintenance Conference, May 20–23, 1997.
10 Kosla, A. and A. Babbini, “Fluid dynamic design of a new generation of recipro-
cating compressor valves,” GE Technology insights, 2013.
11 Lindner-Silwester, T., and C. Hold, “The BCD packing ring—a new high perfor-
mance design HOERBIGER Ventilwerke GmbH & Co KG.” 7th Conference of the EFRC, October 2010, Florence, Italy.
12 XPerSeal Customer Presentation 2013, HOERBIGER.
13 Machu, E. H., “Valve Throttling, Its Influence on Compressor Efficiency and Gas
Temperatures,” International Compressor Engineering Conference, Paper 805, 1992.
MARC VILA FORTEZA is responsible for the rotating machinery/reliability department at Repsol S.A.’s Petronor refinery at Muskiz, Spain, since 2009. He holds an MSc degree in mechanical Engineering and a BSc degree in naval engineering from the Polytechnic University of Catalonia (UPC), Barcelona, Spain. He has published technical articles on reliability, hydrogen compressors and lubrication management.
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with sealless
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