Figure 300-15 shows a PV diagram with an “ideal” valve system. Note that there is no valve loss as shown in Figure 300-16. The valves open and close instantaneously at the exact moment required.
However, in reality, it is virtually impossible to obtain an ideal PV diagram. In an actual compressor cylinder, the valves do not open and close instantaneously; they may lag behind the optimum open or close timing, either due to weak or overly strong springs. For example, if the springs are too stiff, the valve may not remain fully open but will close prematurely. Real valves also present a restriction to flow which, combined with any plates that do not open fully, could cause a greater pressure drop across the valve, thus increasing the power consumption. In addition, volumetric efficiency is reduced.
The compression efficiency of a cylinder depends largely on the valve losses. Manufacturers have made vast improvements in analytical techniques to optimize valve design. Optimized valve designs have effected up to a 15% improvement in Bhp/MM for some applications.
Figure 300-16 is a typical PV diagram showing valve losses. Suction and discharge valve loss is represented by Areas A-F-E and B-C-D. The amount of loss is a function of the flow rate, drag coefficient and mass of the valve elements, valve-spring stiffness, pressure drop, gas pulsations, effective valve flow area, and compressor speed. Therefore, calculation of valve losses is not a straightforward process. It requires a complex computer program with empirical factors related to each specific valve design and cylinder.
Whenever possible, each cylinder should have at least two suction and two discharge valves per end (four suction and four discharge valves total for a double-acting cylinder). A greater number of valves will reduce the effect of a broken valve on cylinder performance and rod loading. In some small bore cylinders, it may be impossible to provide more than one suction and one discharge valve per end. If this is the case, ask the vendor if clearance can be added to a larger cylinder with more valves to achieve the same inlet capacity. In extreme cases, it may be possible to reduce the stroke on a particular crank throw in order to utilize a larger cylinder. Rod loading may be adversely affected, however.
Valve porting also influences volumetric efficiency by contributing to the clearance volume. If the porting must be enlarged to reduce the flow loss, it is done at the expense of clearance volume and a reduction in volumetric efficiency.
Compressor specifications often refer to the average inlet valve velocity as a general index of valve performance. The average inlet valve velocity can be used to make very generalized comparisons of compressor offerings with respect to valve performance. Generally, the lower the average inlet valve velocity, the lower the power loss as a result of valve losses.
The average inlet valve velocity is calculated from the cylinder displacement and the total valve lift area of all the suction valves in the cylinder. The following formula is used to calculate the average inlet valve velocity:
V = average gas velocity, FPM
Vd = cylinder displacement rate, CFM
A = product of the actual lift, valve opening periphery, and the number of suction valves per cylinder, Square Inches
Figure 300-17 shows the lift area in a plate type valve. The product of the actual lift and valve periphery is the valve lift area. These values are furnished by the compressor valve manufacturer. When the valve lift area is not the smallest area in the valve flow path, the average inlet valve area is calculated on the basis of the smallest area.
Note The 288 factor in the valve velocity formula in Paragraph 2.7.1 of API 618 is correct. For many years API, GPSA, and other references erroneously used a constant of 144 in the formula. The 144 was wrong because the original formula was based on all suction valves of a cylinder being open at the same time. Actually, only half the valves are open at any given time in the cycle.
In reviewing bids for compressors, look for large differences in average valve velocities among the proposed machines. Small differences such as 5000 versus 6000 FPM are probably not significant. But if one vendor consistently quotes velocities twice those of another vendor, find out why. Also, determine whether they are quoting “average” or “effective” valve velocities.