VALVES REPORT Flow regulating valve aids chemical injection control subsea
Successful prototype tests indicate potential
for cost savings through fewer chemical hoses
Marcelo Taulois
Kvaerner FSSL
Over the years Kvaerner FSSL has received a number of enquiries concerning the integration of chemical injection systems and subsea control systems. For one recently awarded project, the client specifically requested the control system supplier to control chemical injection flow by installing a flow regulating reducing valve within the subsea control module (SCM).
Mandeville Engineering (MEL), a precision engineering services company part owned by KFL, has been researching and developing such a valve for a number of years. The valve design and fabrication phases having been concluded, the first prototypes are now being qualified by KFL/MEL engineers and are demonstrating excellent performance characteristics.
MEL's reducing valve integrated within KFL's system provides the following operational advantages:
Chemical injection flow rate can be set and fully controlled by operators from the subsea control system master control station.
Automatic injection flow rate adjustment via the control system master control station upon sensing variations of the well pressure.
Cost savings on the umbilical configuration, as fewer chemical hoses would be required. A possible configuration would allow different types of chemicals to be injected through the same umbilical hose and distributed accordingly via the SCM chemical header.
The reducing valve will be located within the SCM as part of the control valve manifold. Chemical supply will enter the SCM via hydraulic couplings and pass through a two-way, two-position isolation valve upstream of the regulating reducing valve before exiting the SCM via an additional hydraulic coupling.
The MEL chemical injection valve is a self-contained unit comprising principally two items: a differential pressure regulating valve and an adjusting actuator. The two items are bolted together as a single package, manifold mounted for the chemical supply and function ports.
Manifold connections have supply and return hydraulic fluid porting for control of the adjusting actuator, although the chemical itself may be used for this role to avoid the use of an additional working fluid.
The adjusting actuator unit needs a 12V DC electrical pulse to activate the solenoids. Solenoid A for clockwise rotation gives increasing pressure and solenoid B for anti-clockwise rotation gives decreasing pressure.
The valve can be fitted with an outlet pressure transducer to confirm the chemical pressure being supplied to the well. Additionally, the regulator back pressure can be recorded by another transducer. With suitable algorithms the flow volume may be accurately calculated.
The differential pressure regulator is a state of the art hydraulic regulator with a sealing poppet. Application of a load via a control spring opens the poppet, permitting flow from the supply. Pressure in the valve sensing chamber acts on a sensing piston which is sealed from the spring chamber. Pressure on the sensing piston acts against the control spring, backing it off and thus permitting the poppet to close off the supply.
An orifice is fitted between the valve sensing chamber and the valve outlet port. A drillway from the outlet port feeds fluid pressure back onto the spring side of the sensing piston. This gives differential pressure control to the regulator with the spring able to give 0 to 200 bar differential (0-345 bar is also available).
The combination of controlled differential pressure across the flow orifice gives an accurate flow control setting which can be varied by adjusting the spring load.
The valve has a sealed, non-rising stem spring adjuster with clockwise motion causing pressure increase. The adjusting actuator is a dual-ratchet torque applicator. The ratchet wheel has two rings of 24 teeth: one ring acts clockwise and the other anti-clockwise. Each ring has its own piston and pawl which are spring energised to a free position.
Two three-way, two-position solenoid valves act to supply fluid to their specific piston. Operation of the A solenoid supplies control fluid to its piston, causing the pawl to stroke the wheel through one tooth in the clockwise direction. De-energisation of the solenoid vents the piston and permits the spring to return the piston to the rest position. The pawl rides up over the ratchet during this return stroke.
Sequential operation of the A solenoid causes an increase in the spring load of the regulator when the devices are fitted together. Sequential operation of the B solenoid causes a corresponding decrease in regulator spring load.
The solenoid valves may be operated using the chemical supply and control fluid provided that a minimum 100 bar differential exists between the supply and function, or at any function pressure provided the vented fluid from the solenoid is permitted to pass to sea or a return line.
The valve testing set-up will include testing upt to 690 bar injection pressure. As part of the qualification testing, actual representative chemicals will be used.
Copyright 1995 Offshore. All Rights Reserved.