How Do They Work?
The Axelson Safety Valve utilizes a differential of areas exposed to the same pressure to maintain a closed position of the plunger assembly. The upper plunger having a greater area than the seat, keeps the safety valve closed when pressures are equal. When the Relief Control reaches its set point, it opens, relieving the pressure on the upper plunger. The line pressure, or the up-stream pressure, raises the plunger assembly off the seat, opening the safety valve reducing the pressure of the system by the Blowdown Control, it opens, feeding pressure into the chamber and returning the plunger to its original seated position.
The Relief Control, is referred to as the RVC, controls the opening, which is actuated by increase in pressure above that for which the control is set to open. The RVC contains a seat with a mating piston which contains a seal that is exposed to the pressure on the Safety Valve. This RVC is maintained in a closed position by the compression of the spring which may be changed by the adjusting screw. To raise the set pressure, turn the adjusting screw clockwise, reverse the direction to reduce the set pressure. When the system pressure reaches the set pressure of the RVC, the control piston is lifted from its seat thereby allowing the pressure above the upper plunger to be reduced as it flows through the drilled passages in the bonnet to atmosphere or on through body passages downstream then to atmosphere. Up stream pressure has now raised the plunger from its seat allowing system pressure to reduce. When the pressure above the upper plunger has been reduced the RVC closes and must remain closed to allow pressure to be applied through the “Blowdown Control” to the chamber above the upper plunger to close the safety valve.
The “Blowdown Control” referred to as BDC, controls the closing of the safety valve. Pressure enters through the side of the BDC through the screen between the two “O” Rings on the outside of the control cylinder; this screen Is in line with a horizontal drilled passage in the bonnet from the bypass tube conducting the upstream pressure to the BDC. (Type 4 BDC has an external tubing connection to the bypass tube). The blowdown balance piston is connected to the blowdown piston by a threaded section through the blowdown seat. Pressure on the Blowdown piston holds this assembly in a closed position preventing the pressure from entering the chamber above the upper plunger until the line pressure is reduced, the pressure on the blowdown piston is also reduced, since they are connected; thereby allowing the spring force to overcome the pressure on the blowdown piston which moves the Blowdown Balance piston off its seat allowing pressure to flow to the chamber above the upper plunger which closes the safety valve.
To Raise the Blowdown pressure (pressure desired for Relief Valve to Close) turn the adjusting Screw clockwise which increases the compression of the spring.
To Lower the Blowdown pressure, turn the adjusting screw counter clock-wise. The shortest blowdown or minimum loss of pressure on the system is obtained by adjusting the BDC to be in the open position when the Relief Valve Control Opens.
When the BDC is adjusted to obtain a long blowdown on the system, from the pressure when the BDC is set to open, to the pressure where the RVC opens, the increase in pressure is conducted through the plunger slide valve to the chamber above the upper plunger to the RVC. As the plunger begins to rise, this flow is cut off as the slotted orifice passes the “O” ring seal.
Relief Valve Control or (RVC)
For all practical purposes this control is a miniature precision-built relief valve. Flow through the relief control seat affects a small area of the relief control piston. When this force becomes greater than the pre-set spring force affecting this piston, it rises, allowing pressure past the seat to affect a larger area of the R.C. piston. And a snap action takes place. Flow through this control is at all times greater than the supply that can be emitted through the slide valve and/or Blowdown Control simultaneously.
This control set to a prescribed pressure allows the Safety Valve to open. The control when opened vents the pressure from atop the plunger assembly creating an unbalanced force condition. Therefore, with the seat force greater the Safety Valve opens.
Blowdown Control or (BDC)
For practical purposes of explanation, we can state that the Blowdown Control is a miniature precision
built back pressure regulator, controlling pressure emitted though the bypass tube or remote-control line. In no way is this control affected by pressure that is above the upper plunger. Pressure enter this control from the side through a fine mesh stainless steel screen. If this pressure is greater than the spring force, it will affect the bottom side of BD piston and the top side of the BD balance piston forcing both upward which would allow the BD seat and O-ring on the bottom half of BD balance piston to seal shut; therefore, preventing flow through BD balance cylinder. If this pressure is less than spring force, the spring will force stem against BD piston and push BD balance piston down and away from the seat so that flow can exist past the BD balance piston and through BD balance cylinder to the top of upper plunger assembly. This control set to a prescribed pressure closes the Axelson safety Valve. If the safety Valve is desired to cycle rapidly, pull the spring down snug causing the BD control to remain open at all times. This control can be field adjusted to allow any amount of blowdown desired.
Under normal conditions, the plunger assembly is held on its seat due to the greater force from the larger area above the plunger. This differential in force keeps the valve seated securely. The volume above the plunger is controlled by three simple elements:
- Plunger Slide Valve Assembly
- Relief Control
- Blowdown Control
The open port in the Plunger Slide Valve provides communication between the inlet of the valve and the relief control so that inlet pressure changes are transmitted directly to the Relief Control.
When the set pressure is reached, the Relief Control opens releasing the pressure in the chamber above the plunger, and the line pressure force moves the plunger assembly to the top of its stroke. Just before the plunger Assembly reached the top of the stroke, the Relief Control closes, trapping sufficient pressure to cushion the opening. The upward stroke of the plunger assembly closes the port in the plunger slide valve.
The Safety Valve remains open until the pressure has relived or “Blown Down” to the set pressure of the Blowdown Control. At this point the Blowdown control opens, feeding line pressure through a filter screen into the cylinder above the Plunger Assembly. Because of the differential area, the force applied above the Plunger Assembly is greater than the force below, and the Plunger Assembly moves toward its seat. Approximately ¼ inch off the seat, the Plunger Slide Valve Port is re-opened and the flow through the Plunger Slide Valve, combined with that through the Blowdown Control, accelerates the Plunger Assembly to its seat without wire-drawing, chatter or hammering. The Valve is now reset, ready for another pressure increase. Only the infinitesimal volume of gas in the chamber above the Plunger is required to actuate the valve. This gas is filter effectively through screens in the Slide Valve Assembly and Blowdown Control.