Valves, Piping & Flow-Control Equipment

Valves, piping, and flow-control equipment determine where water or wastewater can move, how much flow passes through a system, and how equipment can be isolated for operation or maintenance. Operators should understand the basic purpose of common valve types, the difference between isolation and throttling service, and how valve or piping problems affect flow and pressure.

A valve problem may appear to be a pump, pressure, or process problem. Good operators consider the entire hydraulic path before deciding where the fault is located.

Purpose of Valves

Valves are used to control liquid movement through a piping system.

Common purposes include:

• starting or stopping flow;
• isolating equipment;
• controlling flow rate;
• controlling pressure;
• preventing reverse flow;
• directing flow between process paths.

Isolation Valves

An isolation valve is primarily used to fully open or fully close a flow path.

Typical isolation applications include:

• separating a pump for maintenance;
• isolating a tank;
• shutting down a section of piping;
• removing equipment from service.

Some valve types perform better as isolation valves than as throttling valves.

Throttling Valves

A throttling valve is used to intentionally create resistance and regulate flow or pressure.

Throttling changes:

• flow rate;
• pressure drop;
• system resistance;
• pump operating point.

Do Not Assume Every Valve Is Suitable for Throttling

Some valves can be damaged or perform poorly when left partially open for long periods.

Operators should use valve types according to their intended service and manufacturer guidance.

Gate Valves

Gate valves are commonly used for isolation.

A gate moves into or out of the flow path.

When fully open, a gate valve generally provides relatively low resistance.

Gate valves are usually not preferred for routine throttling service.

Gate Valve Problems

Possible problems include:

• stem damage;
• gate not fully opening;
• gate not fully closing;
• corrosion;
• debris in the seat;
• packing leakage;
• operator or actuator problems.

Butterfly Valves

A butterfly valve uses a rotating disc inside the pipe.

Butterfly valves are commonly used for:

• isolation;
• flow control in appropriate applications;
• large-diameter piping.

Because the disc remains in the flow path, even a fully open butterfly valve creates some hydraulic resistance.

Butterfly Valve Position

The disc angle affects flow area and resistance.

A partially closed butterfly valve can create significant head loss.

Operators should verify actual valve position rather than assume a position indicator is always correct.

Ball Valves

Ball valves use a rotating ball with a passage through it.

They are often used for:

• rapid isolation;
• smaller piping;
• chemical systems;
• instrument lines.

Many full-port ball valves provide low resistance when fully open.

Plug Valves

Plug valves use a rotating plug to open or close the flow path.

They are used in some water and wastewater applications and can handle certain solids-bearing services depending on design.

Globe Valves

Globe valves are designed with a flow path that allows effective throttling.

They can provide good flow control but generally create more head loss than a fully open gate valve.

Needle Valves

Needle valves allow fine control of small flows.

They are often used in:

• instrumentation;
• sampling;
• small chemical or air lines.

Diaphragm Valves

Diaphragm valves use a flexible diaphragm to control flow.

They may be used in:

• chemical service;
• corrosive applications;
• slurries;
• systems requiring separation of the actuator from the process liquid.

Check Valves

A check valve allows flow mainly in one direction.

It closes when flow attempts to reverse.

Check valves are commonly used:

• on pump discharge lines;
• in chemical-feed systems;
• where reverse flow could damage equipment or affect process operation.

Check-Valve Problems

Possible problems include:

• stuck open;
• stuck closed;
• debris preventing closure;
• seat leakage;
• slam;
• broken hinge or spring.

Check-Valve Leakage

A leaking check valve can allow reverse flow after equipment stops.

Possible symptoms include:

• loss of pressure;
• pump short cycling;
• reverse flow noise;
• rapid wet-well level rebound.

Check-Valve Slam

If reverse flow develops before a check valve closes, the valve may shut violently.

This can cause:

• banging;
• pressure surge;
• pipe movement;
• equipment stress.

Pressure-Reducing Valves

A pressure-reducing valve, or PRV, automatically reduces downstream pressure to a controlled value.

PRVs are common in distribution systems with significant elevation differences.

Operators may monitor:

• upstream pressure;
• downstream pressure;
• valve position;
• control setpoint.

Pressure-Sustaining Valves

A pressure-sustaining valve helps maintain a minimum upstream pressure while allowing downstream flow.

It may be used to protect:

• source pressure;
• higher-priority zones;
• pump suction conditions.

Pressure-Relief Valves

A pressure-relief valve opens when pressure rises above a selected value.

Its purpose is to protect equipment and piping from excessive pressure.

Repeated relief-valve operation should trigger investigation of the underlying cause.

Control Valves

Automatic control valves can regulate:

• flow;
• pressure;
• level;
• other process variables.

The valve may receive a signal from:

• pressure transmitter;
• flow meter;
• level sensor;
• PLC;
• local controller.

Valve Actuators

Large or automated valves may use actuators.

Common actuator types include:

• electric;
• pneumatic;
• hydraulic.

The actuator provides the force needed to move the valve.

Electric Actuators

Electric actuators use a motor and gear mechanism.

Possible problems include:

• loss of power;
• motor failure;
• limit-switch problem;
• torque trip;
• gear damage;
• control-signal failure.

Pneumatic Actuators

Pneumatic actuators use compressed air.

Possible problems include:

• low air pressure;
• air leak;
• failed solenoid;
• damaged diaphragm;
• sticking valve.

Hydraulic Actuators

Hydraulic actuators use pressurized hydraulic fluid.

Problems may include:

• fluid leaks;
• low hydraulic pressure;
• contamination;
• seal failure.

Valve Position Indicators

Valve position may be shown as:

• open;
• closed;
• percentage open;
• stem position;
• disc angle.

A position signal can fail, so field verification may be necessary when readings do not match hydraulic conditions.

Limit Switches

Limit switches indicate that an actuator has reached a selected position.

They may be used to confirm:

• fully open;
• fully closed;
• another required position.

Torque Switches

Some electric actuators use torque protection.

A torque trip may indicate:

• valve binding;
• debris;
• mechanical obstruction;
• incorrect adjustment.

Do Not Force a Stuck Valve

Applying excessive force can damage:

• stem;
• gearbox;
• actuator;
• valve internals.

If a valve does not move normally, investigate the cause.

Manual Valve Operation

Manual valves may use:

• handwheel;
• lever;
• operating nut;
• portable operator.

Operators should know:

• direction of opening;
• normal position;
• number of turns where relevant;
• whether the valve is normally open or normally closed.

Valve Exercising

Valves that remain in one position for long periods may become difficult to operate.

Planned exercising can help identify:

• sticking;
• corrosion;
• damaged stems;
• inaccurate position records.

Valve exercising should follow facility procedures and consider hydraulic consequences.

Rapid Valve Operation

Opening or closing large valves too quickly can create hydraulic transients.

Possible effects include:

• water hammer;
• pressure drop;
• pressure surge;
• movement of sediment;
• pump operating-point changes.

Piping

Piping carries liquid between process units and equipment.

Important characteristics include:

• diameter;
• material;
• pressure rating;
• wall condition;
• supports;
• joint condition;
• internal roughness.

Pipe Diameter

Pipe diameter strongly affects:

• velocity;
• friction loss;
• flow capacity.

For the same flow, smaller diameter produces higher velocity and generally greater head loss.

Pipe Roughness

Internal roughness can increase because of:

• corrosion;
• scale;
• tuberculation;
• biological growth;
• solids buildup.

Increasing roughness increases hydraulic resistance.

Pipe Supports

Pipe supports control movement and transfer loads to the structure.

Operators should report:

• loose supports;
• broken hangers;
• unexpected pipe movement;
• excessive vibration.

Pipe Strain on Equipment

Piping should not impose excessive force on pumps, valves, or other equipment.

Pipe strain can contribute to:

• misalignment;
• seal problems;
• flange leakage;
• equipment distortion.

Flanges

Flanged joints use:

• flanges;
• gasket;
• bolts.

Leaks may result from:

• damaged gasket;
• loose bolts;
• misalignment;
• corrosion;
• excessive pressure.

Threaded Connections

Threaded connections are common on smaller piping and instrumentation.

Problems may include:

• leaks;
• corrosion;
• damaged threads;
• loose fittings.

Flexible Connections

Flexible connections can accommodate limited movement or vibration.

They should not be used to correct major piping misalignment.

Reducers

Reducers connect different pipe diameters.

They affect:

• velocity;
• pressure;
• flow pattern.

Suction-piping reducer orientation can be important for preventing trapped air.

Elbows and Tees

Elbows and tees change flow direction and create additional head loss.

More fittings generally increase system resistance.

Strainers

Strainers remove debris before it reaches sensitive equipment.

A dirty strainer can cause:

• higher differential pressure;
• lower flow;
• lower downstream pressure;
• pump suction problems.

Differential Pressure Across a Strainer

A rising pressure difference across a strainer often indicates increasing blockage.

The basic relationship is:

Differential Pressure = Upstream Pressure - Downstream Pressure

Flow Meters as Part of the Piping System

Flow meters can also add hydraulic resistance.

Meter problems may include:

• incorrect installation;
• fouling;
• blocked sensing lines;
• signal failure.

Expansion and Contraction

Temperature changes can cause piping to expand and contract.

Systems may use:

• expansion joints;
• loops;
• supports;
• anchors.

Improper restraint can create stress on piping and equipment.

Corrosion

Corrosion can weaken:

• pipe walls;
• valve bodies;
• bolts;
• supports;
• actuator components.

Corrosion can also create leaks and increase internal pipe roughness.

Leaks

Leaks may be caused by:

• corrosion;
• gasket failure;
• loose connection;
• cracked pipe;
• pressure surge;
• vibration;
• freezing.

Leak severity should be evaluated based on pressure, fluid, location, and process impact.

Valve Leakage to Atmosphere

External valve leakage may occur at:

• packing;
• bonnet gasket;
• body joint;
• actuator connection.

Internal Valve Leakage

A valve may appear closed but still pass liquid internally.

Possible evidence includes:

• pressure downstream;
• unexpected tank filling;
• continued flow;
• failure to isolate equipment.

Verify Isolation Before Maintenance

Closing a valve does not prove that the line is depressurized or fully isolated.

Before maintenance, required energy-control and verification procedures must be followed.

Double Block or Additional Isolation

Some maintenance tasks may require additional isolation methods depending on facility procedures and hazard level.

Operators should follow the approved isolation plan rather than assume one closed valve is always sufficient.

Flow-Control Loops

An automatic flow-control loop may include:

• flow meter;
• controller or PLC;
• control valve or VFD;
• flow setpoint.

The controller compares measured flow with the setpoint and adjusts equipment output.

Pressure-Control Loops

A pressure-control loop may adjust:

• control valve position;
• pump speed;
• pressure-reducing valve response.

Valve Hunting

A control valve that repeatedly moves open and closed around the desired position may be hunting.

Possible causes include:

• poor controller tuning;
• oversized valve;
• sticky actuator;
• unstable sensor signal.

Stiction

Stiction is resistance to initial valve movement caused by friction or sticking.

It can cause:

• poor control;
• hunting;
• sudden valve movement after delayed response.

Valve Fails to Open

Possible causes include:

• no actuator power;
• low air pressure;
• failed control signal;
• mechanical binding;
• torque trip;
• incorrect local/remote mode.

Valve Fails to Close

Possible causes include:

• debris in seat;
• actuator problem;
• stem damage;
• control failure;
• mechanical obstruction.

High Pressure Drop Across a Valve

High pressure drop may be expected if a valve is intentionally throttling.

If not expected, possible causes include:

• valve not fully open;
• incorrect position indication;
• internal damage;
• obstruction.

Low Flow Through a Piping System

Possible causes include:

• partially closed valve;
• blocked strainer;
• pipe blockage;
• high downstream pressure;
• pump problem;
• incorrect control signal.

Unexpected Reverse Flow

Possible causes include:

• failed check valve;
• incorrect valve lineup;
• pressure reversal;
• pump shutdown.

Use Pressure Readings to Locate Restrictions

Pressure measurements at several locations can help identify where head loss is occurring.

A large pressure drop across one component suggests that component is contributing significant resistance.

Valve Lineup

A valve lineup is the required set of valve positions for a particular operating condition.

Operators should verify valve lineup during:

• startup;
• shutdown;
• equipment changeover;
• maintenance;
• emergency response.

Incorrect Valve Lineup

An incorrect lineup can cause:

• no flow;
• unexpected flow path;
• overflow;
• loss of pressure;
• equipment damage;
• process upset.

Tagging and Identification

Valves and piping should be clearly identified according to facility practice.

Operators should avoid operating a valve unless they are certain of:

• valve identity;
• system served;
• expected effect.

Update Valve Records

Valve records may include:

• location;
• valve type;
• size;
• normal position;
• direction of operation;
• condition;
• maintenance history.

Common Valve and Piping Mistakes

• Using an isolation valve for continuous throttling when it is not intended for that service.
• Operating large valves too rapidly.
• Assuming the position indicator is always correct.
• Forcing a stuck valve.
• Ignoring check-valve leakage or slam.
• Ignoring rising differential pressure across a strainer.
• Assuming one closed valve guarantees safe isolation.
• Ignoring pipe movement or damaged supports.
• Using flexible connections to compensate for major misalignment.
• Failing to verify valve lineup after maintenance.
• Ignoring actuator air, power, or control-signal problems.
• Changing control-valve settings without understanding process impact.

A Practical Valve Inspection Sequence

1. Confirm valve identification.
2. Verify normal expected position.
3. Check local and remote status.
4. Inspect actuator power or air supply where applicable.
5. Look for external leakage.
6. Check position indication.
7. Compare upstream and downstream pressure where useful.
8. Listen for abnormal noise or vibration.
9. Check for repeated actuator alarms or torque trips.
10. Document abnormal condition.

A Practical Low-Flow Troubleshooting Sequence

1. Confirm the flow measurement.
2. Check pump or source condition.
3. Verify valve lineup.
4. Check for partially closed valves.
5. Review strainer differential pressure.
6. Check upstream pressure.
7. Check downstream pressure.
8. Look for piping restrictions or blockage.
9. Review automatic control-valve position.
10. Compare with normal historical operation.

What to Remember for the Exam

• Isolation valves are primarily used to open or close a flow path.
• Throttling valves intentionally create resistance to control flow or pressure.
• Not every valve type is suitable for continuous throttling.
• Gate valves are commonly used for isolation.
• Butterfly valves use a rotating disc and can create significant resistance when partially closed.
• Globe valves are well suited to throttling but create more head loss than many isolation valves.
• Check valves help prevent reverse flow.
• Check-valve leakage can cause reverse flow, pressure loss, and short cycling.
• Check-valve slam can contribute to hydraulic transients.
• PRVs reduce downstream pressure.
• Pressure-relief valves protect systems from excessive pressure.
• Valve actuators may be electric, pneumatic, or hydraulic.
• A valve-position indication can fail and may need field verification.
• Do not force a valve that is binding or stuck.
• Rapid operation of large valves can cause water hammer and other pressure changes.
• Smaller pipe diameter generally increases velocity and friction loss for the same flow.
• Increasing pipe roughness increases hydraulic resistance.
• Dirty strainers can reduce flow and increase differential pressure.
• Pipe supports, flanges, joints, and flexible connections should be inspected for movement, leaks, and damage.
• Closing a valve does not by itself prove that hazardous hydraulic energy has been isolated.
• Incorrect valve lineup can cause major process and hydraulic problems.
• Pressure measurements can help locate restrictions in piping and flow-control equipment.