Discharge pressure increases

As previously discussed, the solids conveying rate decreases as the discharge pressure increases for all materials studied. Eor HDPE resin, the solids conveying rate decreased almost logarithmically with discharge pressure, as shown in Eig 5.12. This is a characteristic of most crystalline polymers, and has been well documented in prior literature [21] as well as predicted by all of the major models [1,14,19,20, 22, 23]. The HDPE resin demonstrated the greatest solids conveying pellet flow... 

The centrifugal compressor, unless it is dirty or mechanically defective, has to operate on its curve. As the compressor discharge pressure increases, then Hp, the feet of polytropic head required, must also increase. Also, as can be seen from the compressor curve, the volume of gas compressed (ACFM) must decrease. When the volume of gas drops below a critical flow, the compressor will be backed up to its surge point. 

On the other hand, the vibrational temperature of IPS is significantly lower than that of 2PS, as Figs. 8(b) and 4(b) indicate. However, the dependence on the discharge ptressure is qualitatively similar, i.e., the vibrational temperature tends to decrease as the discharge pressure increases. This can also be attributed to the frequent coUisional relaxation with neutral molecules. The discrepancy on vibrational temperatures between IPS and 2PS can be explained by the dominant molecular processes for the excitation and de-excitation of the upper state of IPS and 2PS, that is, B sflg and C Ou. This will be discussed in section 6 in terms of elementary processes in the nitrogen plasma with low discharge pressure. 

Hypercompressors. In an LDPE plant a primary compressor, usually of two stages, is used to raise the pressure of ethylene to about 25—30 MPa and a secondary compressor, often referred to as a hypetcomptessot, is used to increase it to 150—315 MPa (22,000—45,700 psi). The thermodynamic properties of ethylene ate such that the secondary compressor requires only two stages and this results in a large pressure difference between the second stage suction and discharge pressures. 

Dead-heading A blockage on the discharge side of an operating pump which results in the flow reducing to zero and an increase in the discharge pressure. The energy input from the deadheaded pump increases the temperature and pressure of the fluid in the pump. 

Figure 16-7 shows the vibration spectrum with the speed and suction pressure kept constant but with a small 20psig increase in discharge pressure. Notice the large increase in the 9000 rpm component from 0.2 to 1.5 mil (0.0127-0.0381 mm). A further small increase in discharge pressure would have increased the subsynchronous vibrations to more than 1.0 mil (0.0254 mm) and wrecked the unit. 

Proportional-plus-derivative (rate) Moderate Small Any Where increased stability with minimum offset and lack of reset wind-up is required. Compressor discharge pressure... 

One pound of steam at Opsig occupies 1,600 times the volume of a pound of water at atmospheric conditions. This ratio drops proportionately as the pressure increases. When the steam collapses, water is accelerated into the resulting vacuum from all directions. This happens when a steam trap discharges relatively high-pressure flashing condensate into a pump discharge line. 

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. 

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. 

Acamiulation pressure increase over the maximum allowable working pressure of the vessel during discharge through the safety or relief valve, expressed as a percent... 

If the discharge pressure is increased, the amount of slippage will increase. If we increase the pressure in the above example, to 1500 psig, the actual output may drop to 8 gpm. Therefore, the volumetric efficiency will decrease to 80 per cent at 1500 psig. 

Positive-displacement pumps can be divided into two major types rotary and reciprocating. All rotary pumps use some form of rotating element, such as gears, vanes, or lobes to increase the discharge pressure. Reciprocating pumps use pistons or wobble plates to increase the pressure. 

Reciprocating Reciprocating pumps are more difficult to monitor because of the combined rotational and linear motions that are required to increase the discharge pressure. Measurement-point location and orientation should be based on the same logic as that of reciprocating compressors. 

As shown in Example 9-1, increasing the pump impeller size from 13 inches to 13.5 inches increases the flow by 3.8%, discharge pressure by 7.8%, and horsepower by 12%. Increasing the turbine speed from 3,300 rpm to 3,400 rpm increases the flow by 3%, the discharge pressure by 6.1%, and the horsepower by 9.4%. 

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