Suction pressure, compressors

The performance of a centrifugal compressor (discharge to suction-pressure ratio vs. the flow rate) for different speeds is shown in Fig. 11-87. Lines of constant efficiencies show the maximum efficiency. Unstable operation sequence, called surging, occurs when compressors fails to operate in the range left of the surge envelope. It is characterized by noise and wide fluctuations of load on the compressor and the motor. The period of the cycle is usually 2 to 5 s, depending upon the size of the installation. 

In this automatic thrust balancing system, the pressure behind the compressor wheel is controlled to a value between the compressor suction pressure and the wheel peripheral pressure. As the expander inlet pressure increases above the compressor suction pressure level, the resulting thrust force pushes the compressor wheel, and hence the rotor system, towards the compressor suction. In the reverse situation, when the pressure behind the compressor wheel is reduced below the wheel peripheral pressure level, the rotor system moves toward the expander. 

The required suction pressure to the expander is not available until the oxidizer tower reaches design pressure. With two compressors in operation this takes approximately 15 min. Since the expander wheel is mounted on the compressor pinion, enough flow must be available at startup to prevent overheating. The calculated windage loss of the expander wheel (40 hp) requires about 16,000 Ib/lir of flow to prevent overheating. 

The correct flow to use is the compressor suction. However, a flow element such as an orifice in the compressor suction can rob inordinate horsepower. Therefore, sometimes the discharge flow is measured and the suction flow computed within the controller by using pressure measurements. Other times the compressor intake nozzle is calibrated and used as a flow element. The correct AP to use is the discharge minus the suction pressure. 

It is desirable to have additional axial-load control on the multistage compressor. A balance piston, also refeired to as the balance drum, can be located at the discharge end (see Figure 5-46). The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston (opposite the last stage location) is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston area to develop a thrust force opposite that generated by the impellers. The pressure on the... 

In some applications (usually high pressure compressors using oil film seals) alternative pump schemes should be considered. It may be that the desired seal pressure is not achievable by one set of pumps or the quantity required by the seal is small relative to the main pump capacity. There are (imes when booster pumps are needed however, if the reason is energy, it would be worth reviewing the economics very carefully, because reliability tends to suffer with the booster. The booster pumps are paired into a main and standby and are configured to take suction fro lower pressure system. Sufficient interlocks have to be supplied drivers so that if the main pumps shutdown, the boosters come ... 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

As flow rate to the compressor increases, the suction pressure rises until the volume of gas at actual conditions of temperature and pressure compressed by the cylinder equals the volume required by the cylinder. A flare valve is needed to keep the suction pressure from rising too high and overpressuring the suction cylinder, creating too high a rod load or increasing the horsepower requirements beyond the capability of the driver (see Chapter 11 for further discussion). 

A suction pressure throttling valve can also be installed to protect the compressor from too high a suction pressure. This is typically a butterfly valve that is placed in the suction piping. As flow rate to the compressor increases, the valve will close slightly and maintain a constant suction pressure. This will automatically limit the flow rate to exactly that rate where the actual volume of gas equals that required by the cylinder at tlie chosen suction pressure setting. It will not allow the suction pressure to increase and the compressor cylinder to thus handle more flow rate. 

A speed controller can help extend the operating range and efficiency of the compressor. As the flow rate increases, the compressor speed can be increased to handle the additional gas. Compressor speed will stabilize when the actual flow rate to be compressed equals the required flow rate for the cylinder at the preset suction pressure. As the flow rate decreases, the compressor slows until the preset suction pressure is maintained. 

If the suction pressure decreases, and discharge pressure remains constant, the compressor head must increase, approaching the surge point in the process. 

As suction pressure increases or discharge pressure decreases, the compressor head requirement will decrease and the flow rate will increase. A flare valve will avoid stonewalling or overranging driver horsepower. 

The compressor can operate at any point on the performance curve. For the maximum value of suction pressure, the pressure rise across the machine at the surge control point must be less than the system pressure rating. If not, a relief valve should be installed,... 

In mechanical systems, the temperature of the available water (or coolant) to condense the refrigerant from the compressor determines the pressure level of this part of the system. Generally speaking, it is less expensive to operate at as low a pressure level on the discharge as is consistent with the suction pressure and with the physical characteristics of the refrigerant. Sometimes the cost of the refrigerant and the cost of its replacement on loss dictate that the optimum situation is not determined by the system and refrigerant s physical properties. 

A compressor is required to handle 9,360,000 SCFD at a suction of 75 psig and 100°F with the discharge at 300 psig. It is anticipated that the suction pressure may rise to 100 psig after one year of operation. The k value of the gas is 1.3, and the unit will he installed at a coastal installation. 

The method presented here does meet the requirements necessary to ensure a good understanding of the entire system around a compressor. This method has given excellent results from suction pressures of atmospheric to... 

Material Capacity Suction pressure Suction temperature Discharge pressure Discharge temperature Compressor speed Single-cylinder, double-acting... 

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.)...

The FCC reactor pressure is usually controlled at the suction of the wet gas compressor. The reactor pressure is the wet gas compressor suction pressure plus pressure drop through the main fractionator system. 

When the compressor is idle, the lubricating oil may contain a certain amount of dissolved refrigerant, depending on the pressure, temperature, and the refrigerant itself. At the moment of starting, the oil will be diluted by this refrigerant and, as the suction pressure falls, gas will boil out of the oil, causing it to foam. 

If a plant has been shut down long enough for all pressures to equalize and is then restarted, the suction pressure will pull down below normal until the liquid refrigerant has begun to circulate. Under such circumstances the low-pressure switch may operate. This is a normal occurrence, but may require the addition of a delay timer to prevent frequent starting of the compressor motor. 

Back pressure regulation valves (Figure 9.5) can be used in the suction line, and their function is to prevent the evaporator pressure falling below a predetermined or controlled value, although the compressor suction pressure may be lower. 

Pressure drops on the high-pressure side will be small enough to have little effect on the performance of the complete system. Pressure losses in the suction pipe and its fittings, especially if this is long, should be checked, and a correction made for the actual compressor suction pressure. For low-temperature applications, pipe sizes may have to be increased to avoid excessive frictional losses at these low pressures. 

A natural gas (methane) pipeline is to be designed to transport the gas at a rate of 50,000 scfm. The pipe is to be 6 in. ID, and the maximum pressure that the compressors can develop is 10,000 psig. The compressor stations are to be located in the pipeline at the point at which the pressure drops to 100 psi above that at which choked flow would occur (this is the suction pressure for the compressors). If the design temperature for the pipeline is 60°F, the compressors are 60% efficient, and the compressor stations each operate with three stages and interstage cooling to 60°F, determine... 

A compressor feeds ethylene to a pipeline, that is 500 ft long and 6 in. in diameter. The compressor suction pressure in 50 psig at 70°F, the discharge pressure is 800 psig, and the downstream pressure at the end of the pipeline is 300 psig. For each of the two following cases, determine (1) the flow rate in the pipeline in scfm and (2) the power delivered from the compressor to the gas, in horsepower ... 

Natural gas (CH4) is transported through a 6 in. ID pipeline at a rate of 10,000 scfm. The compressor stations are 150 mi apart, and the compressor suction pressure is to be maintained at lOpsig above that at which choked flow would occur in the pipeline. The compressors are each two stage, operate adiabatically with interstage cooling to 70°F, and have an efficiency of 60%. If the pipeline temperature is 70°F, calculate ... 

Ethylene (MW = 28) is transported from a storage tank, at 250 psig and 70°F, to a compressor station where the suction pressure is lOOpsig. The transfer line is 1 in. sch 80, 500 ft long, and contains two ball valves and eight threaded... 

A natural gas pipeline (primarily CH4) is supplied by a compressor. The compressor suction pressure is 20psig, and the discharge pressure is 1000 psig. The pipe is 5 in. sch 40, and the ambient temperature is 80°F. 

The variables. Each pipeline segment has associated with it five variables (1) the flow rate Q (2) the inlet pressure pd (discharge pressure from the upstream compressor) (3) the outlet pressure ps (suction pressure of the downstream compressor), (4) the pipe diameter D, and (5) the pipeline segment length L. Inasmuch as the mass flow rate is fixed, and each compressor is assumed to have gas consumed for operation of one-half of one percent of the gas transmitted, only the last four variables need to be determined for each segment. 

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