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Valves Activities (Functions and Maintenance): Control Valves, PSV’s, PRV’s,MOV’s, MV, HP Valves & Steam Traps

Valves and their functions

Valves are devices that direct, regulate and control flow of fluids such as gases, liquids and fluidized solids. They vary amount of flow in fluids, relieve components, and regulate downstream system. They open, close, and obstruct passageways partially. Check valves prevents and checks fluids flowing in one direction.

Valves have a body and bonnet. The body forms outer cover containing internal parts. The bonnet is the part through which stem passes. The body is made of metallic, brass, cast iron, alloy steels, gunmetal, bronze, and steel. Ports allow passage of fluid through valves. Valve member and discs regulate flow by obstructing Bonnets (Den, 2016). The disc is an obstruction that is movable in the stationary body regulating flow inside the valve. The seat forms the interior surface of the body contacting the disk forming a tight seal to avoid leakage.

 The stem is responsible of transmitting motion from controlling device to the disc. Gaskets are mechanical seals preventing fluid leakage from valves. Valve balls is used in high-tolerance applications and heavy duty. Springs are used in loading to shift disks in various positions by default, but still controlled with an aim of repositioning the disk. Relief valves use springs to shut valves but at the same time permit excess pressure to open valves against spring loading. Valve actuator is involved with operation of disk and stem assembly. Actuators are manually operated hand-wheel, manual lever, motor operator, pneumatic operator, and hydraulic ram (Den, 2016). Valve packing is used to prevent leakage from spaces left between bonnet and stem. Fibrous materials are used for packing such as flax, Teflon. It is compressed to prevent loss of fluid and damage of stem valves.

Valve maintenance

Slight leakages are maintained by tightening eyebolt nuts using equal force on both sides. Addition of gland packing in the stuffing box is done through rotating hand wheel in anticlockwise direction all in full open position. Globe and gate valves are availed a back seating arrangement in total full open position. An angular scriber is used to remove old packing and replace it with new correct sizes with open ends placed at 180 degrees with each other. The spindle is cleaned and lubricated since it is located above area meant for packing, exposing it to open atmosphere with much dust and harsh conditions of weather. Smooth valve operation is maintained through protecting the spindle-threaded portion. Lubrication of the spindle is done periodically with grease gun applying grease (Valvulas, 2009). Generally valves should be stored properly away from contamination with hard particles in the pipeline.

Types of valves

Control Valves, PSV’s, PRV’s, MOV’s, MV, HP Valves, & Steam Traps

Control valves

Control valves are widely used in processing plants to regulate pressure, levels of fluids, temperature within desired range. Control valves manipulates flowing fluids such as chemical compounds, water, gas and steam with an aim of compensating load balance and regulate variable processes. They consist of a sensor, transmitter, and controller to compare variable process. The controller sends correct signals to final controlling element. Control valves regulate pressure and rates of flow in fluids (Solken, 2016). Globe valves are used for control because of their flanged ends that ease maintenance. Control valve modulates flow by moving valve plug as related to ports in the valve body.

 Pressure control valve (PCV) acts as the initial line of defense for primary safety.

PCV is a device that acts proportionally opening and closing to increases in pressure. PCV controls generation of excessive pressure found in fluid line thus used in applications that require reduced pressure to cylinders between valve output and cylinder. It is used in hydraulic and pneumatic circuits to prevent production of excess pressure. PCV increases passage of flow to reduce pressure and decrease C/S area in cases of increasing pressure. Damaged PCV negatively affects actuation of hydraulic component and fluid lines. Example, PCV controls fluid pressure flowing to the cylinder or hydraulic actuator (Fukui seisakusho Company, 2016). High volume check valves is installed to allow reverse flows rapidly.

Pressure relief valve (PRV)

Pressure relief valve (PRV) is the relief device on a vessel filled with liquid acts as the final move to protect vessel and fluid line from bursting. The valve does not open suddenly but proportional according to increases in pressure. It opens fully after sensing excess pressure to relieve excess fluid, avoiding disasters, and accidents. it does not release excess pressure until the pressure in the vessel exceeds designed criteria (Fukui seisakusho Company, 2016). PRV operates is in operation when there is excess pressures and remains idle when the pressure is rising in the vessel. However, damaged PRV destroys equipment that generates pressure and injures workers around. Example, boilers having PRVs do not generate excess pressure in boiler vessel. Spring loaded PRV expands when axial load is over designed pressure-causing opening of valves so that excess pressure is relived. Relief valves are used when the drop in line pressure is higher than 3% of set point and backpressure is over 50%.

Types of pressure relief valves include

Conventional relief valves

Conventional pressure relief valves (PRVs) operations are directly affected by changes in back pressure. Use of conventional pressure relief valves is prohibited when built-up backpressure is higher than 10% of set pressure at 10% over pressure. Built-up backpressure allowable at higher maximum is used for excess pressure higher than 10%. Conventional relief valves are reliable and versatile in cases of proper sizing. However, it has a negative effect on backpressure when valve is releasing pressure, thus accumulating pressure in protected equipment.

Balanced bellows relief valves

Incorporation of bellows minimizes backpressure effect on valves operational characteristics. Bellows surround area equal in size to inlet orifice area, which is free and well ventilated from backpressure effect on the relief valve discharge side. Allowed pressure of relief valves is 10-50% of set pressure. Balanced bellows relief valves have no backpressure effect during accumulation of pressure and its release (Fukui seisakusho Company, 2016).The only disadvantage is its release of flammable and toxic substances to the atmosphere through vents found in bellows.

Pilot operated relief valve

Pilot valve is a small valve used for safety and has a spring. Smaller pilot valves that are self-actuated control and combined main relief in setup operated by a pilot. Relief valves makes use of process fluid that is circulated via pilot valve in application of closing force on safety valve disc. Operation of pilot operated valves is assisted by chosen arrangement having minimal margin between pressure set on relief valve and operating pressure found in protected equipment (Fukui seisakusho Company, 2016). The only disadvantage is blocking of inlet of the pilot valve tubes by wax, ice and hydrate. Vacuum relief admits external fluid preventing excess vacuum in the internal.

Pressure safety valve (PSV)

PSV is a relief device installed on compressible fluid and vessels filled with gas. The valves open suddenly and fully after reaching set pressure. Conventional safety relief valve the opening characteristics are affected by superimposed backpressure (Fukui seisakusho Company, 2016).

Steam traps

Steam traps are automatic valves that is used in filtering condensed steam, gases that are non-condensable including air without allowing the steam to escape. Industries use steam to heat and mechanical power among other uses that prevent loss of steam. The steam trap valve is self-contained to automatically drain condensate from enclosures having steam and prevents loss of steam. Moreover, it permits controlled flow off steam at an adjustable rate. Extra steam traps allow passage of non-condensable gases and avoiding escape of live steam. Steam traps allow consumption of steam in negligible amounts (Fluid control institute, 2008).

Steam trap mechanism

Steam trap mechanism are operation principles developed to automatically discharge condensed and uncondensed gases. Common mechanisms rely on differences in pressure, gravity, and temperature.

Types of steam traps

Mechanical traps

Mechanical traps have rising and falling floats depending on levels of condensate, with an attached mechanical linkage, which gradually opens and closes valves. There is direct relationship in operation of mechanical traps and levels of condensate in steam trap body. They have a three years’ service life. Mechanical traps include float traps and inverted bucket (Fluid control institute, 2008).

Temperature traps

Temperature traps valves are driven off or on through expansion or contraction from differences in temperature. Condensates are removed soon after formation to reduce emissions of carbon dioxide. Condensates are removed at temperatures below forty degree centigrade. Examples include thermostatic traps, Bi-Thermostatic traps and bimetallic traps.

Thermodynamic (TD) traps

TD traps perform when there are differences between dynamic response and changes in velocity of flows in incompressible and compressible fluids. Entering of steam, the disk is forced against the valve seat by static pressure over the disk. Static pressure in wide area overcomes the steam high inlet pressure. Condensation of the steam leads to reduction of pressure existing against the disk and trap cycles, making TD trap into a time cycle device that opens in presence of steam (Fluid control institute, 2008). As a result, premature wear occurs and shutting of trap in case the non-condensable gas is trapped in the disk.

Venturi nozzle traps

Venturi nozzle traps wok depending on two-phase flow while removing condensate from systems of steam since steam and condensate undergo many stages. It has complicated sizing that does not allow escape of steam. Eddy currents that are erosive are eliminated and contaminants carried along with condensate. Condensate chocks nozzle protecting from steam loss during removal of condensate.

Metering valve (MV)

Metering Valve (MV) control fluids to actuate and govern different types of equipment and mechanisms. Their scale of color on stem is exclusive; the flow is controlled in both directions and has micro-fine needles. The styles are straight and right angled and straight. They have a metal-to-metal sealing, torque seated and its quarter design is triple eccentric. There are no leakages while seats do not rub each other (Orton, 2009). The valves are fire tested thus being fireproof. MV valves are used to process fluids, hydrocarbons, geothermal steam, sour and hot gas, hydrogen, and oxygen, abrasive service, acid, chloride, and caustic services as well as sulfur recovery. The temperature is limited to -196 degree centigrade to 818 degrees centigrade. The pressure is limited from full vacuum to 450 Bar. Materials used to manufacture it include superduplex, Monel, Inconel, Alloy 20, titanium, aluminum, and bronze.

High performance (HP) Valve

HP valves have double offset seat/stem and disc geometry that provides super sealing with reduced seat contact and wearing out. It has many sealing edges that are machined and tapered. The shaft is a single piece that is designed to provide maximum strength and CV. Its internal position of the cast disk is designed to stop preventing excess travel of the disk and maintain optimum contact of the disc and seat. The stem packing is adjustable and centered equipped with oversize bearings that reduce leakage of stem and other emissions that are fugitive. There are disk spacers that place the disc in center of the seat to avoid seating distortions. The proof stem is blown out to avoid injury and loss of products (Milwaukee valve company, 2010). The pressure and mechanical design combined is repeated to get reliable sealing performance with prolonged life cycle. The connection of the stem and disc only permits minimal losses of motion and strength. Examples are P series Lug with lever 21/2”-8”,HP series Lug with Gear 21/2”8”, HP series Lug with Gear 10”-24”,HP series Wafer with lever 21/2”-8”, and HP series Wafer with Gear 21/2”-8”.

Motor operated valve (MOV)

Motor operated valve (MOV) is important in piping and plant system. MOV valve are large in size and applicable in many ways such as pump discharge. The motors fully open or close valves in pipelines. Example cooling water lines that process pipelines that do not require fluid control. Motor operated valves are used to totally stop or allow flow of fluids. The valves are not applicable in throttling purpose since they serve off-on service application. MOV are of many types including gate, butterfly, and ball valves all having actuator control (USNRC Technical training center, 2010). Electric motors often mounted on the valve, geared to the stem of the valve to ensure closing or opening of the valve when motor is operating. Motor operated valves are applied during frequent operations, and when locations of valves is hazardous, inaccessible and in remote places.

Open/close valves

 Open/close valves automate manual open close valves like sanction valves, pump discharge, boiler feed water isolation valves and product line valves.

Itching valves

Itching valves require degree of control such as gradual opening and closing. It is applied in IBD valves, boiler main steam valves, boiler start up vent, reflux lines (USNRC Technical training center, 2010).

Precision flow valves

Users enable continuous control in Precision flow valves by making use of feedback from field to controller that is unavailable in other valves operated by the motor. Examples include injection valves.

References

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