POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO AUTOMATED CONTROL VALVES & PRESSURE REGULATORS W EEK - 6 09:29 1.

POWER EQUIPMENT

INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO

AUTOMATED CONTROL VALVES & PRESSURE REGULATORS

WEEK - 6

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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.

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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

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DEGREES OF AUTOMATION Fully automated

controlNo direct human

intervention needed

Semi-automated controlPeriodically

requires human intervention Part automated –

Part manual

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AUTOMATION CLASSIFICATIONTHREE BROAD CATEGORIES

Position controlPosition control An elevator

Speed controlSpeed control Rotating equipment like turbines, diesels etc.

Process controlProcess control Regulating of temperatures, pressures, fluid flow,

tank levels, density control

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AUTOMATION CLASSIFICATIONSIMPLE ‘DISPENSING’ SYSTEM Process control valves include

the three categories: Dispensing

Seem like flow control Dissipating

Seem like pressure control Distributing

Seem like direction control

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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

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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

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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.

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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.

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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.

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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.

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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

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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.

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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

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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

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VALVE ACTUATORS TYPES Piston (cylinder) actuators – uses a piston to

position the valve disc. Most common application is ball and

butterfly valvesMore expensive than diaphragm valvesGreater internal frictionVery small and quite

powerful for their size

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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

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VALVE ACTUATORS TYPES

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

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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.

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ACTUATOR PROBLEMS Leaking diaphragms – causes

Poor reaction to signal changeExcessive air consumptionControl valve will not stay in positionNote – a leaking air line from a pilot

controller to the valve diaphragm can cause the same symptoms.

Valve stem sticking – causesPoor valve reactionStem sticking can be cause by

Packing gland to tight Stem worn or damaged Corrosion inside the valve

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PRESSURE REDUCING AND REGULATING VALVES Pressure reduction is

achieved by placing a restriction in the lineA 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.

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PRESSURE REDUCING AND REGULATING VALVES There are two ways to

automate pressure reduction.Multi component

automated control loopsSelf contained pressure

regulating valves

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PRESSURE REDUCING AND REGULATING VALVES Multi component control

loopsMinimum 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.

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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.

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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  

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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.

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PRESSURE REDUCING AND REGULATING VALVES The principal parts:

An upward-seating main valve 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.

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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.

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PRESSURE REDUCING AND REGULATING VALVES

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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.

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THANK YOU

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