October 28, 2008

Type of Valves - Ball Valve

In the last article I discussed brief & important things about Globe Valve. Today I will discuss some basics of Ball Valve.



The Ball valves as name suggests is having a ball type spherical body. They are stop valves that use a ball to stop or start the flow of fluid. The ball performs the same function as the disk in the globe valve.

Ball Valves
This rotational-motion valve uses a ball-shaped disk with a hole bored through to stop or start fluid flow. When the valve handle is turned to the open position, the
ball is rotated so that the hole lines up with the valve body’s inlet and outlet.

When the ball is rotated so the hole is perpendicular to the flow, the valve is closed. Because the ball moves across the seats with a wiping motion, ball valves can handle fluids with suspended solids.

When the valve is shut, which requires only a 90-degree rotation of the handwheel for most valves, the ball is rotated so which is divided in two parts by a baffle. Flow direction is steeply changed in this type of valve so the controlling of flow is better by the movement of restriction element.

Most ball valves are of the quick-acting type (requiring only a 90-degree turn to operate the valve either completely open or closed), but many are planetary gear operated. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves contain a swing check located within the ball to give the valve a check valve feature. Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.

Ball valves are available in Venturi, reduced and full-port patterns. The full-port pattern has a ball with a bore equal to the inside diameter of pipe. Most ball valves instead have a reduced bore with a Venturi shaped flow passage of about three quarters the nominal valve size.

CONCLUSION
Uses a large lever to turn a ball that closes or opens the flow with one quick quarter turn.

Ball Valves are mainly used in fluids containing solids.

Ball Valves have lower pressure drops.

Are the standard for natural and LP gas, replacing the older plug valves that were traditionally used as gas valves. Now Ball Valves are standards for gas service also replacing Plug Valves.

Available in either metal or plastic, threaded or non-threaded types.

Ball valves with double-stem seals provide greater durability.

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October 21, 2008

Vacuum Pumps - Dry Vs Liquid Ring

Vacuum is any system of reduced
pressure, relative to local (typically atmospheric) pressure. Achieved with a pump, vacuum systems are commonly used to

• Remove excess air and its constituents.
• Remove excess reactants or unwanted byproducts.
• Reduce the boiling point.
• Dry solute material.
• Create a pressure differential for initiating transport of material

Liquid-ring and dry pumps offer the most advantages for the chemical process industries (CPI). Both of these pump types have bearings sealed off from the pumping chamber and do not require any internal lubrication because the rotors do not contact the housing. Both, when employing a coolant system, prevent the coolant from contacting the process fluid and causing contamination, and both use mechanical shaft
seals for containment.

Liquid Ring Pumps
In the cylindrical body of the pump, a sealant fluid under centrifugal force forms a ring against the inside of the casing (See Figure below).



The source of that force is a multi-bladed impeller whose shaft is mounted so as to be eccentric to the ring of liquid. Because of this eccentricity, the pockets bounded
by adjacent impeller blades (also called buckets) and the ring increase in size on the inlet side of the pump, and the resulting suction continually draws gas out of the vessel being evacuated. As the blades rotate toward the discharge side of the
pump, the pockets decrease in size, and the evacuated gas is compressed, enabling its discharge.

The ring of liquid not only acts as a seal; it also absorbs the heat of compression, friction and condensation. Popular liquid choices include water, ethylene glycol, mineral oil and organic solvents.

Advantages
  1. Simpler design; employs only one rotating assembly.

  2. Can be fabricated from any castable metal.

  3. Little increase in the temperature of the discharged gas.

  4. No damage from liquid or small particulates in the process fluid.

  5. Maintenance and rebuilding are simple.

  6. Slow rotational speed (1,800 rpm or less), maximizing operating life.

  7. Can use any type of liquid for the sealant fluid in situations where mingling
    with the process vapor is permissible.

  8. No lubricating liquid in the vacuum chamber to be contaminated.

  9. Accommodation of both condensable vapors and noncondensables, while operating as both a vacuum pump and condenser.

Disadvantages
  1. Mixing of the evacuated gas with the sealing fluid.

  2. Risk of cavitation requires a portion of process load to be noncondensable under operating conditions.

  3. High power requirement to form and maintain the liquid ring, resulting in large motors. Therefore consume more energy.

  4. Achievable vacuum is limited by the vapor pressure of sealant fluid at the operating temperature. Hence require Chilled water & hence operating cost is higher than dry pumps.


Screw Pump
Rotary-screw pumps dominate as dry pumps in the CPI, particularly in larger-size pump applications. Other type of dry pumps are Rotary-claw and rotary-lobe.

Two long helical rotors in parallel rotate in opposite directions without touching, synchronized by helical timing gears. Gas flow moves axially along the screw without any internal compression from suction to discharge. Pockets of gas are trapped with in the convolutions of the rotors and the casing, and transported to the discharge.




Compression occurs at the discharge port, where the trapped gas must be discharged
against atmospheric pressure. Each convolution of the rotor acts similarly to a stage in series with the one behind it; at least three convoluted gas pockets in the rotor are required to achieve acceptable vacuum levels.

See the figure & saving nos in my previous article - Guaranteed Energy Saving in Vacuum Pump.

Advantages
  1. Rugged rotor design, constructed of sturdy cast or ductile iron without any flimsy rotating components.

  2. Noncontact design facilitated by timing gears.

  3. High rotational speed reduces the ratio of gas slip to displacement, increases net pumping capacity and reduces ultimate pressure.

  4. Multiple staging provides inlet pressures below 1-mm Hg absolute while discharging to atmosphere.

  5. No contamination of evacuated gas & so process vapors can be recovered thru downstream condensation. Very cost effective in such situations.

  6. Due to lack of condensation, pump can be fabricated of standard, inexpensive cast iron.





Disadvantages
  1. Cannot handle particulate matter, nor large slugs of liquid.

  2. May discharge gases at high temperatures.

  3. Most difficult to repair or rebuild.

  4. May require a gas purge for cooling, or to protect the bearings and seals from the process gas.

  5. Due to high operating temperatures,some process gases may polymerize.


Source: CHE Fact Sheet

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October 14, 2008

Type of Valves - Globe

In the last article I discussed brief & important things about Gate Valve. Today I will discuss some basics of Globe Valve.



The Globe valves as name suggests is having a globe type spherical body which is divided in two parts by a baffle. Flow direction is steeply changed in this type of valve so the controlling of flow is better by the movement of restriction element.

Globe Valves
A Globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. Like Gate valve which is the most common for isolation type service, Globe valves are most common for flow regulation service.

Globe Valves are used when a valve must be opened and closed frequently under high water pressure. They are used to control volume of flow. These valves have two chambers with a partition between them for passage of water that must change course several times from port to port.

Globe Valves should not be used in water supply lines for occasional shut-off purposes.

CONCLUSION
Globe valves are not designed for general shut off conditions.

Globe valves are used for controlling of flow by reducing the pressure.

Angle Valve is a kind of globe valve which changes the flow direction by 90° thus eliminates need of elbow & conventional globe valve combination.

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October 07, 2008

Type of Valves - Gate Valve

It is very difficult for fresh engineers to understand the use of different kind of valves according to the process requirement. In which condition we should use a particular type of valve? What are the differences in each one of them.

To simplify the matter, I have compiled some information from different resources, which can be used by all of you.

Although many different types of valves are used to control the flow of fluids, the basic valve types can be divided into two general groups: stop valves and check valves.

Besides the basic types of valves, many special valves, which cannot really be classified as either stop valves or check valves, are found in the engineering spaces. Many of these valves serve to control the pressure of fluids and are known as pressure-control valves. Other valves are identified by names that indicate their general function, such as thermostatic recirculating valves.

Here I am discussing few basic types of stop valves and check valves.

Gate Valves
Gate valves are by far the most widely used in industrial piping. That's because most valves are needed as stop valves - to fully shut off or fully turn on flow - the only job for which gate valves are recommended.

Gate valves are used when a straight-line flow of fluid and minimum restric­tion is desired. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve.



Gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.

Gate valves provide optimum performance in conditions where high flow efficiency, tight shut off and long service is required.

Gate Valves are designed to operate fully open or fully closed. Because they operate slowly they prevent fluid hammer, which is detrimental to piping systems. There is very little pressure loss through a gate valve. In the fully closed position, gate valves provide a positive seal under pressure.

A gate valve usually requires more turns - more work - to open it fully. Also, unlike many globe valves, the volume of flow through the valve is not in direct relation to number of turns of handwheel.

CONCLUSION
Gate valves, while not designed for throttling or too frequent operation are generally ideal for services requiring full flow or no flow.

Gate valves are not designed for throttling.

In a slightly opened position high-velocity flow will cause wire drawing and erosion of seating surfaces in gate valves.

Repeated movement of disc near point of closure under high-pressure flow may gall or score seating surfaces on downstream side.

Slightly opened disc in turbulent flow may cause troublesome vibration and chattering.

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