Parallel compressors

Figure 13-12. Parallel compressor cylinders with surge drums.

Two parallel compressors can be loaded to keep both of them at equal distance from their surge curves. 

Similar structures can be developed for the pressure control of a common discharge or suction header for N parallel compressors. 

Split-range control discusses several interesting practical systems used in chemical processes, such as several boilers discharging into a common steam header, and parallel compressors discharging into a common header. 

By adding parallel compressors which are running warm, so as to ensure instant switch over capability we are introducing two new parallel compressors. The plot plan of the suggested design is shown in Figure 11. 

Following HAZOP of this modified design it was found that additional failure cases were present at the new compressors. Due to the need for immediate switchover to the parallel compressors, the equipment items would be at process conditions and therefore pose identical hazards as the operational compressors. In the QRA duplicate failure cases are used to represent loss of containment from the kick-back loops around each compressor. As this is a simplified case study no other changes were made to the QRA. 

Table 3. Risk ranking at escalation location after introduction of parallel compressors. 

Figure 12. Risk contours for case with parallel compressors.

Offer standby (cold) redundancy, so as to reduce the potential for full pressure leaks from the parallel compressors—this would have an adverse but tolerable effect on the production efficiency... 

Parallel flow, suction in ends Series flow (basic compressor)... 

Capacity control is more complex in compressor networks where two or more compressors operate in series or parallel. Automatic distribution of the load between compressors is required. The antisurge and load sharing control loops must be coordinated to ensure surge protection while tlie load distribution and energy consumption are optimized. 

Instead of using conventional refrigeration and condensers to convert incoming gas into LNG, the system was designed to feed 2,340 Nm / day (86,800 scfd) into each of two expanders. Both are connected in parallel, with compressors attached at the back end. The compressors are connected in series to boost a single stream of returning gas. 

Most centrifugal compressors in service in petroleum or petrochemical processing plants use vane less diffusers. A vaneless diffuser is generally a simple flow channel with parallel walls and does not have any elements inside to guide the flow. 

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup. 

Rgure 3-26. Mantfbld-fype volume txnttles are used where cylinders are operated in parallel, as on this two-stage, motor driven compressor. (Courtesy of Dresser-fland)... 

The steep pressure-volume curve permits the axial compressor to oper ate very well in parallel with other axial compressors. The pressures do not have to match precisely to permit load sharing, as the steepness of the curve allows for adjustment without danger of going into surge or taking wild load swings as sometimes happens when attempts are made to opci ate centritiigal compressors in parallel,... 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

Calculation of the specific work and the arbitrary overall efficiency may now be made parallel to the method used for the a/s cycle. The maximum and minimum temperatures are specified, together with compressor and turbine efficiencies. A compressor pressure ratio (r) is selected, and with the pressure loss coefficients specified, the corresponding turbine pressure ratio is obtained. With the compressor exit temperature T2 known and Tt, specified, the temperature change in combustion is also known, and the fuel-air ratio / may then be obtained. Approximate mean values of specific heats are then obtained from Fig. 3.12. Either they may be employed directly, or n and n may be obtained and used. 

This result requires some explanation. An argument was given by Denton [6], who pointed out that the expansion of the mixed gas (I + ip) from to may be considered as a combination of unit flow through the turbine from T3 to T4, and an expansion of a flow of (/(from T2 to 7), through a reversed compressor (Fig. 4.2). The cycle [1,2,3,5,6,1] of Fig. 4.2a is equivalent to two parallel cycles as indicated in Fig. 4.2b a cycle [1,2,3,4,1] with unit circulation plus another cycle passing through the state points [1,2,2,1] with a circulation tp. The second cycle has the same efficiency as the first (but vanishingly small work output) so that the combined cooled cycle has the same efficiency as each of the two... 

But this expansion through the LP turbine may be considered as two parallel expansions. The first is of mass flow (1 + from the temperature Tg to a temperature Tg (a continuation of the expansion of (1 + i/(h) from 5 to 9) and the second is of mass flow i/(l through a reversed compressor from state 7 to state 1 (which cancels out the... 

C — la Three compressors of identical size operat-C — lb ing in the same process service, connected C — Ic in parallel. 

Figure 12-46D, Part 1. View toward inlet of 4 Vj-in. diameter brazed aluminum impeller. Note Regardless of the metal of manufacture, enclosed impellers with back-leaning blades are extremely useful in applications requiring a steep head-volume characteristic and the highest attainable efficiency. Applications include parallel operation with other compressors or boosting of another compressor s output. The power-volume curve will show a self-limiting feature at higher volumes. This feature is very beneficial when the driver has limited power available but operation throughout the full capacity range is required. (Used by permission A C Compressor Corporation.)...

Figure 13-1 A. Parallel acting compressor cylinders with common suction and discharge surge bottles or drums.

This arrangement is based on compressor cylinders operating in parallel and being of the same size and characteristics. 

Example 13-1. Surge Drums and Piping for Double-Acting, Parallel Cylinder, Compressor Installation... 

Use either a 40-in. l.D. header for these three compressors (two running or equivalent with three) or bring in two parallel headers, cross tied, of about half the flow area each, which equals (5,210) (3.408)/ (2,000) (2) or 4.53 fti. This corresponds to a pipe diameter of 2 ft, 5 in., say 2 ft, 6 in. This is still a large pipe, and space arrangements may dictate which is preferred. If this header is coming to the compressors from a great distance, the pressure drop must be checked to be certain that the system drop will ensure specified pressure at the suction of the cylinders. In some cases, the header size can be made slightly smaller if the pressure... 

Figure 14-33. Gas engine driven parallel compression cylinders in process gas plant service. Note that the front side of gas engines are on the right with high-pressure compressor cylinders extending horizontally left. Also note the suction side pulsation drums on top of compressor cylinders, mid-way. (Used by permission Cooper-Cameron Corporation, Reciprocating Products Division.)...

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