POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO

POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO AUTOMATED CONTROL VALVES & PRESSURE REGULATORS Week - 6

Learning Objectives • Understanding of the fundamentals of automation, classification, and terms associated with automatic valves. • Identify the types of actuators and control valves used in automation systems. • Determine the three categories of control valves. • Define the functions of the control valve actuator, application and limitations, troubleshooting and repair of actuators. • Identify the types of pressure reducing valve and operating principles associated with each type.

Automated Valves • Why automate? • Advantages: • Increases process efficiencies • Reduced manpower/maintenance • Increase safety • Quicker more reliable control • Increase mechanical advantage • Disadvantages • Greater initial cost • Complexity, more personal training • Higher qualification of workers

Degrees of automation • Fully automated control • No direct human intervention needed • Semi-automated control • Periodically requires human intervention • Part automated – Part manual

Automation ClassificationThree Broad Categories • Position control • An elevator • Speed control • Rotating equipment like turbines, diesels etc. • Process control • Regulating of temperatures, pressures, fluid flow, tank levels, density control

Automation ClassificationSimple ‘Dispensing’ System • Process control valves include the three categories: • Dispensing • Seem like flow control • Dissipating • Seem like pressure control • Distributing • Seem like directioncontrol

Automation ClassificationSimple ‘Dispensing’ System • Process control valves - three categories: • Dispensing • The control valve dispenses steam or water for functions like temperature control. • A lube oil cooler will have a valve control valve to regulate the flow of the cooling fluid through the cooler to maintain a constant temperature • A superheater attemperator control valve dispenses saturated steam to the superheated steam to control the final temperature

AUTOMATION CLASSIFICATION SIMPLE ‘DISSIPATING’ SYSTEM • Dissipating • Pressure dissipation • Pressure regulation falls into this category • The automated valve regulates system pressure, high pressure inlet and a controlled lower pressure outlet

Automation ClassificationSimple ‘Distributing’ Valve • Distributing • Dividing a process flow into separate lines or tanks • The main slide valve has two coupled shells. • The inner shell is the actual distributing valve. • In the fully left position, it connects the left cylinder end with the channel B and the right cylinder end with the channel A. • When the main slide valve moves to the right by the action of the eccentric, it closes both ports.

Terms and definitions associated with automated valves • Valve capacity • Rate of flow through a valve • Dead Band • The amount of diaphragm pressure change that will not bring about valve stem movement. • A good way to think of dead band in mechanical systems is to consider the lost motion in a connecting linkage due to excessive bushing or pin wear. • Excessive friction can also cause dead band to occur. • This is also known as lost motion. • In some control systems, we do not want small variations in the feedback signal to move the stem. • Most automated valves will allow the operator to set the sensitivity of the valve, in other words set the dead band.

Terms and definitions associated with automated valves • Feedback signal • I ’s the measured signal that determines what the valve has to do. • In a system in which pressure is being regulated, as the controlled pressure is rising, the feedback tells the valve to move in the closed direction. • Diaphragm Pressure Span • The low and high actuating signals that the diaphragm valve operates. • For example, • a diaphragm pressure control valve may be controlling system pressure to 25 psi. • To regulate this signal, the diaphragm will receive a pneumatic air signal from the air pilot controller from 5 psi to 25 psi. • From this situation, the diaphragm pressure span is 20 psi.

Terms and definitions associated with automated valves • Direct or Indirect Acting • Refers to is the control air signal to the valve is to the top or bottom of the diaphragm. • In direct acting valves the signal is on the top of the diaphragm and increasing air pressure causes the diaphragm and actuator stem to move downward. • In indirect or reverse acting actuators, the signal is to the bottom of the diaphragm and increasing air pressure causes upward movement of the actuator stem.

Terms and definitions associated with automated valves • Fail Open • The valve opens wide if an air pressure failure occurs • Fail Closed • The valve closes if air pressure failure occurs • Fail Safe • The valve remains the in a fixed position when air pressure failure occurs

Terms and definitions associated with automated valves • Closed Loop System • The valve process output is measured and sends a feedback signals to a controller. • The controller compares this signal to the set point signal, and then tells the valve what to do (open more, …… ). • This is a closed loop system. • Open loop system has no feed back signal • Transducer • A device that changes a pressure or temperature signal into an electrical signal.

Valve Actuators Types • A valve actuator is what causes the position of the seat to actually move. • Some common actuators are: • Diaphragm actuators • Piston actuators • Electric motor actuators • Solenoid actuators • Hydraulic actuators

Valve Actuators Types • Diaphragm actuators – very common in steam/condensate systems • By using large diaphragms, a very small air signal to the diaphragm can have huge mechanical advantage to move the valve disc. • Direct acting – signal is on top of diaphragm • Indirect acting – signal is to the bottom of the diaphragm

Valve Actuators Types • Piston (cylinder) actuators – uses a piston to position the valve disc. • Most common application is ball and butterfly valves • More expensive than diaphragm valves • Greater internal friction • Very small and quite powerful for their size

Valve Actuators Types • Electric motor actuators – they are usually used when high speed valve movement is necessary or long strokes are required in large valves. • Relatively low torque outputs. • Very expensive

Valve Actuators Types • Solenoid Actuators – used for on/off applications only and are quite small.

Valve Actuators Types • Hydraulic actuators – use hydraulic fluid to position the valve. • Requires a complete hydraulic system • Very expensive • Very powerful for their small size.

Actuator problems • Leaking diaphragms – causes • Poor reaction to signal change • Excessive air consumption • Control valve will not stay in position • Note – a leaking air line from a pilot controller to the valve diaphragm can cause the same symptoms. • Valve stem sticking – causes • Poor valve reaction • Stem sticking can be cause by • Packing gland to tight • Stem worn or damaged • Corrosion inside the valve

Pressure Reducing and Regulating Valves • Pressure reduction is achieved by placing a restriction in the line • A simple pressure reduction can be accomplished by placing an orifice in a line. • If either the inlet pressure varied or the demand changed on the low pressure side, the pressure would vary. • To keep the regulated pressure constant, you need to vary the orifice size.

Pressure Reducing and Regulating Valves • There are two ways to automate pressure reduction. • Multi component automated control loops • Self contained pressure regulating valves

Pressure Reducing and Regulating Valves • Multi component control loops • Minimum requirements • Pressure sensing device • Pilot pressure controller • Actuated valve • The control pilot controller receives a signal from the reduced pressure side of the system and sends a operating signal to the diaphragm control valve.

Pressure Reducing and Regulating Valves • Multi component control loops • Two types of valves are • An upward seating single valve disc • A downward seating single valve disc. • Both of these valves are single seated unbalanced valves • They are the least expensive valve seating arrangement. • The valve can have a single seat which is balanced • Balance seats are arranged so high pressure acts on both sides of the seat which allows the valve to be operated with less effort.

Pressure Reducing and Regulating Valves • Multi component control loops • Valve can also have two discs on one valve stem, which are also balances. • It is called a double seat pressure balanced plugs. • The high pressure medium enters between the two seats and exerts opposing forces on each of the seats

Pressure Reducing and Regulating Valves • Self contained pressure regulator has all of the control components needed in one valve. • Automatically reduce supply pressure to a preselected pressure • As long as the supply pressure is at least as high as the selected pressure.

Pressure Reducing and Regulating Valves • The principal parts: • An upward-seating mainvalve that has a piston on top of its valve stem, • An upward-seating auxiliary valve, • A controlling diaphragm, • An adjusting spring and screw. • The pressure entering the main valve assists the main valve spring in keeping the reducing valve closed by pushing upward on the main valve disk.

Pressure Reducing and Regulating Valves • Some of the high pressure is bled to an auxiliary valve. • The auxiliary valve controls the admission of high pressure to the piston. • The piston has a larger surface area than the main valve disk, • Open the main valve. • The auxiliary valve is controlled by a diaphragm.

Pressure Reducing and Regulating Valves

Pressure Reducing and Regulating Valves • The control system below shows a supply service arrangement. • The variable pressure sensing line is actually measuring the pressure exerted by the column of water above the sensing point of the variable pressure line to the water level. • As the water level changes, the pressure will vary in the variable line.

THANK YOU

POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO