This feature is especially useful w

This feature is especially useful where it may be difficult to make drift realignment.Base plate ring 70 may be rotated without changing the drift to accommodate the piping to inlet 74.FIG.5 is a schematic disclosing numerous attachments that can be made to upper plug 50 and inlet valve 74 of actuator 10.Upper plug 50 preferably includes a substantially large diameter threaded outer connection to avoid stresses when using accessories.Clear stem protector 116 protects top shaft 12 from adverse effects of weather, sandblasting, contaminating operating environments, and painting.Heat sensitive lock open device 118 mechanically holds open the actuator and valve when other safety systems are inoperative.This device locks the device in the down position allowing it to rise only in the event of fire.Mechanical override 24 is used to mechanically stroke the valve, and is preferably used on low pressure valves or during installation and testing.Electrical limit switch contact 20 permits remote indication of gate valve position.Various types of fusible plugs 122, quick exhaust valves 124, pneumatic relays 126, and other sensors 128 may be used with inlet 74 and top stem 12.In the operation of diaphragm-type actuator 10 of the present invention, pressure is applied through fluid port 74 which moves both diaphragm 40 and diaphragm retainer plate 48 axially towards valve body 60.This movement engages downstop 54 to move bonnet stem 22 downward(towards valve body 60)until downstop 54 contacts stem spacers 92, whereupon further downward movement of bonnet stem 22 is prevented.At this point, gate 22 is properly aligned so that the valve is open(assuming a normally configured gate valve).If pressure is lost or purposely evacuated, the valve is closed via pressure from spring 58 acting against downstop 54 to move bonnet stem 22 axially away from valve body 60 until metal-to-metal contact is made at secondary stem seal 112.This action is referred to as fail-closed operation.If required, the valve can be configured with a fail open gate design for vent or blow-down systems.FIG.6 discloses a dual actuator system which may be used to double the stroke power.Secondary operator 140 preferably threadably attaches to plug 143 via connector 141.Lock down plug 154 prevents rotation of operator 140 with respect to operator 10.Operation of secondary operator 140 is similar to that of single actuator 10.Pressurized fluid enters fluid port 142 causing diaphragm plate 144 to move downwardly, thereby forcing stem adaptor 146 and top shaft 148 downwardly.Air is vented from vent hole 150 during the down stroke.Downstop 152 controls the down stroke drift in the manner discussed hereinbefore.FIG.7 discloses a hydraulic valve actuator 200 embodiment of the present invention.Top shaft 202 is kept clean via rod wiper 204 disposed within removable top plug 206.Dual wear bearings 208, preferably formed of MOLYGARD®, are used to support top shaft 202.