Surge curve

On a plot of P vs. h, the surge curve becomes a straight line. 

When two compressors operate in series, one can be dedicated to maintain the total discharge pressure (PIC-01) from the pair, while the speed of the other can be manipulated to keep them at equal distance from their surge curves. This is achieved by FFIC-02 controlling the distribution of the total compression ratio between them. 

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

Controlling the two compressors (FCP-2 and RPC-3), which are operating in parallel, requires the use of load distribution controls. Section 2.5.4 (Chapter 2) describes how two compressors can be proportionally loaded and unloaded, while keeping their operating points at equal distance from their surge curves (see Figures 2.19 and 2.22). 

Depending on the relationship between pressure drop and flow caused by the equipment and controls downstream of the blower, an increase in blower speed could either cause a blower to surge, or stop it from surging. The shape of the surge curve, which is unique to each compressor, also influences the tendency of a machine to surge as its speed is altered. 

Fig. 4. Selection of fan size where the soHd line represents a typical setting and the dashed lines the operating extremes, (a) Desirable sizing. The system resistance curve intersects the fan curve near its maximum efficiency. Changes in system resistance from a flow-control element also intersect the fan curve at desirable points for good flow control. The dashed curves also intersect system resistance curves at desirable locations, (b) A fan essentially too large for the system. The intersection of the system curve near the peak of the fan curve results in poor system flow control and perhaps surging.

With these data, one can determine the dielectric curve the machine must have when switched with such an interrupting device. This can be compared with the actual dielectric curve of the machine (Figure 17.18) obtained from its manufacturer to decide the compatibility of the interrupting device for the machine or vice versa and the extent of surge protection, if necessary. For more details and results of similar simulation tests, see Central Board of Irrigation and Power (1995). 

Curve-Oa, - Original steep fronted TRV, r.r.rv. = — Curve-0a2 - Damped TRV2 with the use of a surge arrester alone, r.r.r.v. may still be higher than recommended tor Va... 

Curve-0b2 - Tamed and damped TRV with the use of a surge capacitor and a surge arrester... 

Q (4) Represent amplitude and steepness of the arriving surge at different locations corresponding to curves of Figure 17.21. 

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 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,... 

Figure 10-14. Typical centrifugal compressor curve showing surge.

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

A speed controller is needed in conjunction with the surge control sy tern. A new head-capacity curve is established for each speed, as shown in Figure 10-14. 

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,... 

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