In the past decade a new type of actuator has been developed, which use the principle of the reactive gas turbine – a jet engine — as the driving force. The actuator has therefore been named the jet engine actuator. This article discusses the advantages of the jet engine actuator (supplied by low pressure compressed air at 0.4MPa) in comparison with the piston pneumatic-hydraulic actuator.
By Elena Uryvaeva, General Director KITEMA Ltd., and Vadim Sayapin, Doctor of Technical Sciences, professor of Moscow Aviation Institute, Moscow, Russia
The maximum starting torque M n on the shaft of the piston pneumatic-hydraulic actuator (Figure 1) is specified as: M n = P·S n ·L (1) where P – gas pressure in the pneumatic cylinder cavity; S n – piston area; L– lever of actuator’s power transmission. The maximum starting torque of the jet engine actuator (Figure 2) is specified as: M nc =K·G c ·n·v c ·q (2) where K – constant coefficient; G c = P o ·μ·A kc gas discharge through the drive nozzle; P o – gas pressure on inlet of the nozzle; μ – discharge coefficient of the nozzle; A kc – nozzle critical crosssection area; n – number of the nozzles in the rotor of the jet drive actuator (here n = 2); v c — gas flow velocity from the nozzle; q — gear ratio of actuator’s power transmission.
In Figure 2 the distance between the axis of rotation of the jet engine and the nozzle axis is designated as l. It can be seen from the description above that, in order to maintain the initial starting torque of the piston actuator, it is necessary to increase the piston area S n or the level L when the gas pressure is decreasing at the actuator inlet. In consequence, that would necessitate a complete design change and an increase in the overall dimensions and weight of the actuator.A pressure of 8 -16 MPa, compressors and high pressure receivers are need for piston actuators, working on compressed air (without design changes).
Jet drive actuators have a working pressure of 0.4 – 1.6 MPa (on the drive inlet), which allows the actuator to be supplied with compressed air at relatively low pressures. The gas discharge through the nozzle Gc and its flow velocity from the nozzle Vc depend on gas physical parameters with the invariable nozzle parameters and for supercritical mode of gas flow from the nozzle are calculated by the following formulas:
where: k – adiabatic exponent of the operator body; R – gas constant; T o — gas temperature on the nozzle inlet; P c — gas pressure on the nozzle cross-section (we assume that P c = P amM — environment pressure).
From formulas (1) and (2) we get:
The adiabatic exponent for natural gas is k g =1.3, for air k a =1.4. That means that when the jet engine goes over to compressed air operation the ratio of moving force is equal to the ratio of numerical value of square roots in the right part of equation (3), calculated for compressed air and natural gas.
The pressure ratio P c /P o=0.15 when P o= 0.66 MPa (real pressure in a pneumatic system).This is why the ratio of actuator torque for air and natural gas is M a /M g = 1.019, thus there is no significant change in the actuator torque. When natural gas is changed to compressed air, the jet engine actuator torque practically remains constant, which is confirmed also by long standing working experience.That is why the actuator needs no adjustment when natural gas is changed to compressed air in the same range of the working pressure (0.4 1.5 MPa).
Gas pressure calculations
From formula (1) it can be seen that it is necessary to increase the piston area or the length of the lever or both, so that products S L will increase 20 times to retain the torque value while decreasing the inlet pressure of the working gas from 8.0 to 0.4 MPa.This is practically the same as creating a new actuator design.
At the same time the working pressure of the jet engine actuator is the inlet pressure of the drive P ax. P ax = 0.4 1.6 MPa (6) This pressure, which is independent of the pressure in the gas pipeline, is supported by the pressure regulator built-in into each actuator’s electro-pneumatic control device. The value P ax is determined individually for each type and size of ball valve, depending on the time required to cycle the valve. From formulas (2), (3) and (4) is evident that to ensure ensuring the same nozzle force it is necessary to increase the critical cross-section of the nozzle by a factor of 2.4 – 3, that is to increase its diameter by a factor of roughly 1.7, keeping the same configuration of the nozzle.
All jet engine actuators are equipped with changeable nozzles, which allow them to be used in various conditions. The use of spare nozzles was included in the design concept of the actuator from the very beginning. The aim was to ensure the actuators could deliver additional torque or be used in further applications when utilized in typical pneumatic systems. It is important to note one more important advantage from using compressed air at a constant pressure of P ax =1.0 MPa as the power source with a jet engine actuator. Namely that a gas pressure regulator is not required in the electro-pneumatic control device of the actuator. Its role is effectively met by a serial electro-pneumatic valve, which greatly simplifies the actuator’s design and reduces.
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