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Saving Compressor power (TIA)

An Indian refinery has cut the power consumption of a reciprocating compressor on a hydrocracker by 86 kW and saves approximately €55000/y on energy costs, while simultaneously increasing reliability, by replacing the valves with a modern profiled plate design.

Andreas Horinek
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Article Summary
The refinery in north-eastern India is rated to process 3 million t/year of Assam Mix crude containing
0.26 wt% sulphur. The main products are LPG, diesel, jet fuel, and high grade kerosene.

There are two hydrocrackers, each rated at 1.45 million t/y. The unit in question uses Chevron technology and was commissioned in 2000. The hydrogen make-up compressor is a three stage machine running at 300 rpm with cylinder lubrication and a rated power of 3300 kW. Suction pressure is 19 bar and discharge pressure is 
186 bar.

The compressor was originally fitted with traditional non-metallic plate valves supplied by Hoerbiger. The refinery operator wanted to save power and increase valve reliability while retaining the ability to operate with nitrogen as well as hydrogen.

Hoerbiger recommended the company’s new XP profiled plate valve. 136 mm XP valves were installed on the first and second stage cylinders. The third stage cylinder has 136 mm and 147 mm XP valves at the crank and head end respectively.

Since the new valves were installed in October 2015 they have run without problems and no maintenance has been needed. The operator reports an average power saving of 86 kW thanks to the high efficiency of the profiled plate design.

The profiled plate valve is a proven design that cuts valve losses by up to 40% while giving up to six times the service life of conventional valves (see Figure 1).

The Hoerbiger XP valve is targeted at API compressors handling technical gases such as nitrogen, natural gas, and a wide range of refining and petrochemical applications including LDPE, syngas, carbon dioxide, and hydrogen-rich gases. It is suitable for both lubricated and non-lubricated compressors at speeds up 
to 1500 rpm, and is available in diameters of 
86–261 mm.

Compressor valves have huge influence on process reliability and energy use. Unsuitable valves can waste 10% of the energy consumed by the compressor because of gas dynamic losses and frequently also sticking caused by excess oil. Moreover, although many valves run trouble-free for a year or more, lifetime in difficult duties may be measured in days.

Unfortunately, energy efficiency often conflicts with reliability. High efficiency requires a large open area and high lift. However, large open areas require narrow sealing elements, while high lift increases the velocity of the valve plate. Both factors make the valve more prone to breakage.

The XP valve combines advanced materials and manufacturing methods with aerodynamic profiles, and an optimised spring design. The result is a valve that is highly efficient and above all robust.

Materials, design, and manufacturing
Modern engineering polymers have replaced steel valve rings and plates in high performance applications. Polymers are lightweight and resist impact and fatigue loads well. However, they are tricky to mould to 
the tight tolerances needed, and differential thermal expansion can be a problem when sealing against steel seats.

To solve these issues, Hoerbiger developed PowerPEEK, a lightweight carbon reinforced composite that combines the optimal stiffness for good sealing with exceptional impact resistance. PowerPEEK has a coefficient of thermal expansion similar to that 
of steel, and resists high temperatures, oil, and oxidation. Components can be manufactured to tight 
tolerances via a patented net-shape moulding technique that orients the reinforcing fibres for maximum strength. This allows the use of narrow sealing elements with profiled edges for lower frictional losses (see 
Figure 2).

Profiled valve components reduce oil stiction thanks to reduced contact area and a pressure induced “preload” that aids rapid opening.
Profiling of the plate and the seat also avoids stress concentrators, and so allows closing velocities up to twice as high as for non-profiled designs. Additionally, dynamic load resistant coil springs improve valve reliability. Additionally, dynamic load resistant coil springs improve valve reliability.

These new profiled plate valves are ideal for 
the demanding conditions found in modern compressors. The trend towards fewer, larger valves and
higher operating speeds enables compressors with lower capital costs and smaller footprints, but 
places greater demands on valve reliability. So too does the growing use of stepless capacity control systems that hold the valves open for part of the compression cycle.

This short case study originally appeared in PTQ's Technology In Action feature - Q3 2017 issue.

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