Waste Heat Recovery On Multiple Low-Speed Reciprocating Engines
Authors: Mayhew Ronald E. 
Published: December 2007

Mayhew, Ronald E.; SPE; Exxon Co. U.S.A.

Summary
With rising fuel costs, energy conservation has taken on added significance. Installation of waste heat recovery units (WHRU's) on gas turbines is one method used in the past to reduce gas plant fuel consumption. More recently, waste heat recovery on multiple reciprocating compressor engines also has been identified as having energy conservation potential. This paper reviews the development and implementation of a WHRU potential. This paper reviews the development and implementation of a WHRU for multiple low-speed engines at the Katy (TX) gas plant. WHRU's for these engines should be differentiated from high-speed engines and gas turbines in that low-speed engines produce low-frequency, high-amplitude pulsating exhaust. The design of a WHRU system must take this potentially pulsating exhaust. The design of a WHRU system must take this potentially destructive pulsation into account. At Katy, the pulsation forces were measured at high-amplitude frequencies and then used to design a pulsation filter and structural stiffness into the various components of the WHRU to minimize vibration and improve system reliability.

Introduction
The Katy gas plant operated by Exxon Co. U.S.A. is located 15 miles [24 km] west of Houston. The plant is designed to dehydrate and process up to 960 × 10 6 cu ft/D [27 × 10 6 m3/d] of gas using a refrigerated lean-oil-type process to recover ethane, propane, and butanes-plus products. Both process to recover ethane, propane, and butanes-plus products. Both compressed and high-pressure field gases are dehydrated and then processed before they enter the sales pipelines (Fig. 1). The heat-transfer medium used in the process plant is a heavy aromatic heating oil. The WHRU provides supplemental heat from the exhaust of six 3,300-hp [2460-kW] provides supplemental heat from the exhaust of six 3,300-hp [2460-kW] integral compressors. During a 1-week test, the WHRU reduced plant fuel consumption by 500,000 cu ft/D [ 14 × 10 3 m3/d], consistent with the design. Much of the development work for the WHRU and subsequent modifications involved equipment that experienced cyclic loading, which included vibration caused by resonance or insufficient wall thicknesses. Cyclic loading is merely a dynamic force causing deflection at some frequency. Resonance is the condition or phenomenon of a vibrating system that responds to a harmonic force with maximum amplitude and occurs when the frequency of the harmonic force is the same as the natural frequency of the vibrating object. This loading can be induced acoustically or mechanically. In the WHRU, mechanical resonance could result from the transmission of engine vibration, while acoustical resonance may result from exhaust pulsation forces. The solutions to these problems were (1) to add mass and structural support to the vibrating components of the system and (2) to install a pulsation filter upstream of the heater to reduce the pulsation amplitudes. pulsation amplitudes. System Process and Mechanical Design
The heat-transfer fluid used in the process plant reboilers is a heating oil or hot oil with a specific gravity of 0.99 at 60F [16C] (Fig. 2). This oil is circulated at 10,000 gal/min [633 dm3/s] and heated in two 496 × 10 6-Btu/hr [145-MW] process heaters. They recover waste heat from two 20,000-hp [14.9-MW] industrial gas turbines in addition to burning up to 4 × 10 6 cu ft/D [0.11 × 10 6 m3/d] of fuel. The WHRU oil circulation rate of 1,500 gal/min [95 dm3/s] is about 15% of the total plant hot oil flow. The oil enters the 22.4 × 10 6-Btu/hr [6.56-MW] waste heater at 450F [232C] and exits at 515F+[268C] after receiving supplemental heat from 478,000 lbm/hr [60.3 kg/s] of 670F [354C] engine exhaust. Six 3,300-hp [2460-kW], two-cycle, 275-rev/min GMWE engines provide exhaust to the convection waste heater. The exhaust is directed from each engine towards its respective 30-in. [76-cm] diverter tee valve allowing exhaust to enter the duct header during normal operation (Fig. 3). Each diverter valve automatically switches its respective exhaust flow to a muffler (the fail-safe position) during startup and when an engine shuts down. The diverter valve operates like two butterfly valves within a tee that are connected by a common linkage. It consists of two disks supported by shafts, which are operated together by a pneumatic spring return actuator. Diverter valves do not provide complete shutoff, so another valve is required to ensure isolation from the WHRU during engine maintenance. Knife gate valves were installed between each diverter valve and the exhaust duct header to prevent this exhaust backflow. The knife gate valves are opened and closed by manually operated air motors. The exhaust duct header varies in diameter from 30 to 60 in [76 to 152 cm]. From the header a transition tee connected the ducts to the waste heater. The original rectangular tee in the duct to the heater was removed later when a pulsation filter was installed. In addition to the existing bellows-type expansion joints at each engine exhaust, expansion joints of a different design were installed in the duct header between each engine and in the main duct upstream of the heater.
JPT
p. 1552