Operating points

NPSHA is an abbreviation of net positive auction head available, it is calculated by Eq. (4). It is a measure of the liquid pressure available at the pump suction to suppress the vaporization or cavitation inside the pump. A hydraulic analysis of pump suction piping will determine the NPSHA. For conservative, NPSHA is usually calculated based on minimum liquid level or maximum suction lift at maximum flow. 

NPSHR is an abbreviation of net positive suction head required. It is determined by experiments at pump manufacturer s laboratory. Per Hydiaulic Institute Standard, NPSHR is the measui NPSHA w hen pump head is dropped 2% below its value on the performance curve. NPSHR increases with higbet pumping rate, higher pump speed, and for some pumps with smaller impeller [1]. 

There are four fetors that cause a ceutrifugal pump tQ he kss ellicient They are [1] 

Hydraulic losses These are liquid tnclioual lo s at impeller, diffuser, and casing wall. 

Volumetric losses These are caused by leaking liquid from pump discharge to the suction side. 

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For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

Once /3 is known, the resistance value for each step can be determined. In the above case, the slip corresponds to the slip at the operating point of the torque or current curve, where the desired torque or current /max occurs, This can be obtained from the torque and current curves available from the motor manufacturer (Figure 5.4). 

The rated discharge is at a static head of /7j and a motor h.p. P. In the process of controlling the discharge from Q to and Qy, the valve is throttled, which increases the head loss of the system (or system resistance) from A/ji to and H,yy respectively. The operating point on the curve now shifts from point A[ to /It... 

The operating point shifts to near point 4 and beyond on the curve. It may conduct a current of 2..5-20 kA or more, during a very fast-rising voltage surge. 

The operating point 1 temporarily shifts to near point 3. Now also the current is also resistive and in the range of a few mA. 

Aris and Amundson (1958) solved the coupled, time-dependent material and energy balances, linearizing the equations about the operating point by a Taylor series expansion. This made the solution possible by the method of characteristic equations. The solution yielded two equations, one the slope condition and the other recognized by Gilles and Hofmann (1961) as the condition that sets the limits to avoid rate oscillation. This is called the... 

Step 3. The final selection of a specific air blower and turboexpander must be made after fully considerating the results of Steps 1 and 2. The normal operating point must be located on the system operating map (similar to Figure 4-66) so that reasonable latitude is available in operating variables between normal operating point and air blower minimum flow, and between operating point and expander bypass point. 

The shaft shall be eapable of transmitting the maximum power at any operating point on the data sheet. The shaft end shall have a j in./ft taper for a hydraulie-fitted eoupling hub (Figure 5-19). 

An expander performanee map of shaft power versus mass flow with lines of eonstant inlet temperature and pressure shall be provided. There shall be a minimum of four eonstant pressure lines with inerements of approximately 5 psi and a minimum of four eonstant temperature lines with inerements of approximately 100°F. The map shall be valid for rated speed with normal exhaust pressure and gas eomposition. The normal operating point shall be indieated on the map. 

The set point of the antisurge eontroller is represented on the eompressor map by a line parallel to the surge limit line. The eontroller ealeulates the deviation of the operating point from this surge eontrol line by the equation ... 

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