The Dynamics Of Reciprocating Compressor Valve Springs

The valves in reciprocating compressors use springs to control the timing of the valve closing. These springs are subjected to dynamic loading by the motion of the valve element. The element motion is periodic, but includes rapid acceleration and deceleration. It can therefore excite a wide range of frequencies in the spring. The resulting spring surge creates high stresses and is a major contributor to premature spring failure. Valve failures are the most common reason for unscheduled compressor shutdowns. The valve springs are the most common source of these valve failures. Therefore, valve spring life is of utmost importance in attaining increased compressor reliability. This paper discusses two approaches to calculating the spring stresses caused by the surge. The first is an approximate method that considers only torsional deflection of the spring wire and neglects end coil effects. The second is a more complete analysis that uses both kinematics and FEA simultaneously to replicate the spring’s response to the dynamic loading. The results of the two methods are compared and their usefulness in preventing spring failures is discussed. INTRODUCTION The valves considered here are used in process compressors. Each application is different and the optimum valve design must be selected for every order. The valve type, the valve lift and the springing are strongly dependent on the gas molecular weight, the compressor speed, the cylinder size and the operating pressures. Each compressor may experience different operating conditions from day to day depending on process requirements. In addition, the gas is often dirty, abrasive, corrosive and wet. Selection of the spring material and design to meet the corrosion and endurance limits is often the most difficult part of the process. The spring stress is frequently much higher than that predicted by a simple static analysis. To get accurate values, the spring surge caused by the rapid motion of the sealing element must be considered. This over stress has been calculated for valves in which the flat sealing element also acts as the spring, but is not normally calculated for the coil springs considered here (Adams; Futakawa; Moaveni; Woollatt). In many practical cases, the spring dynamics are such that adjacent spring coils clash, which can cause mechanical damage. Methods to predict the severity of this coil-to-coil contact are also required. It is also probable that the ends of the spring will temporarily leave their stops. This is a possible contributor to wear and should be predicted. The calculation requirements listed above can be met by a modern finite element method that includes kinematics. However, as the natural frequency of the spring dynamics is high compared to the time period of interest, the finite element analysis can take a considerable amount of time to complete. It must be repeated for the suction and discharge valves of each cylinder under each operating condition. In practice, based on time and resources, it is not economical to perform such an in depth analysis for every order. Therefore a simpler, less accurate, method that can be combined with the valve dynamics prediction that is done for each application is needed. This simpler or approximate method of calculating spring stresses adds only a negligible amount of time to each run. THEORY Approximate Method The approximate method for calculating spring dynamics considers only torsion of the wire and ignores the effects of closed end coils. It does take coil-to-coil contact into consideration and can allow the end coils to leave their stops. With the assumption that only torsional deflection of the wire is important, the basic equation (Wahl 25-7) is: t y b s y a t y ∂ ∂ − ∂ ∂ = ∂ ∂ 2 2 2 2 2 where y = Deflection of Wire in Direction of Spring Axis t = Time a = Speed of Torsion Wave in Wire s = Distance along Wire b = Damping Factor (Twice Wahl’s definition) From Wahl (5-18)