The variable-gradient design-line method is a widely accepted procedure for spacing gas-lift valves (GLV's) in a continuous-flow gas-lift (GL) installation. Injection-pressure-operated (IPO) and production-pressure-operated (PPO) GLV's can be used in a variable gradient designed installation. The primary purpose of GLV's is to unload a well to the desired depth of gas injection. If the installation design is based on a constant surface injection-gas pressure (pio). the GLV's must be opened by an increase in the flowing-production pressure at valve depth (ppfD) rather than an increase in injection-gas pressure at valve depth (pioD). PPO, also called fluid-operated, valves are opened and closed by changes in ppfD.
This paper outlines in detail the calculations for a variable-gradient continuous-flow installation design procedure based on a constant pio for spacing the unloading PPO valves. The valve spacing and port size selection includes performance characteristics of PPO GLV's. A simplified method for calculating the injection daily volumetric gas rate (qgsc) throughput of an unbalanced bellows type of PO valve on the basis of a change in ppfD and the valve bellows- assembly load rate (Blr) is given in the Appendix.
Pressure-operated GLV's can be either IPO or PPO valves. There are major differences in GL installation design considerations for these two types of GLV's. The primary opening force for the IPO GLV is an increase in pioD, and an increase in ppfD provides the primary opening force for the PPO valve.
Most published methods for designing continuous-flow GL installations are based on static force balance equations. The increase in pressure, or pressures 1 to stroke the valve stem is not considered in these design calculations. Design safety factors are included in the GLV mandrel spacing and opening pressure calculations to compensate for the lack of GLV injection qgsc throughput information.
An understanding of the PPO GLV's injection qgsc throughput performance In relation to a changing PpfD is important for PPO GL installation design and GL operations analyses.