Tuesday, July 19, 2016

Operating Principle - Solenoid Valve

Solenoid magnetic field
A solenoid is an electric output device that converts electrical energy input to a linear mechanical force.

At the basic level, a solenoid is an electromagnetic coil and a metallic rod or arm. Electrical current flow though the coil produces a magnetic field, the force of which will move the rod. The movable component is usually a part of the operating mechanism of another device. This allows an electrical switch (controller) to regulate mechanical movement in the other device and cause a change in its operation. A common solenoid application is the operation of valves.

solenoid valve basic parts
Solenoid valve basic parts
A plunger solenoid contains a movable ferrous rod, sometimes called a core, enclosed in a tube sealed to the valve body and extending through the center of the electromagnetic coil. When the solenoid is energized, the core will move to its equilibrium position in the magnetic field. The core is also a functional part of valve operation, with its repositioning causing a designed changed in the valve operating status (open or close). There are countless variants of solenoid operated valves exhibiting particular operating attributes designed for specific types of applications. In essence, though, they all rely on the electromechanical operating principle outlined here.
A solenoid valve is a combination of two functional units.

  • The solenoid (electromagnet) described above.
  • The valve body containing one or more openings, called ports, for inlet and outlet, and the valve interior operating components.

Flow through an orifice is controlled by the movement of the rod or core. The core is enclosed in a tube sealed to the valve body, providing a leak tight assembly. A controller energizing or de-energizing the coil will cause the valve to change operating state between open and closed, regulating fluid flow.

Share your control valve requirements and challenges with an application specialist. Combining your process application knowledge with their product expertise will produce the most effective solutions.

Wednesday, July 13, 2016

Eight Process Control Valve Selection and Application Criteria

Sliding Gate Control valve with actuator
Sliding Gate Control Valve
Schubert & Salzer
Fluid processes will employ control valves to regulate flow or pressure in between the extremes of fully open and fully closed. Their function and design is specifically different from shutoff valves, which are designed and intended for isolation of segments of a fluid system. Improperly applying or sizing a control valve can have consequences in operation, productivity, and safety ranging from nuisance level to critical. Here are some items that should always be part of your selection  and application consideration.

  • A control valve is not intended to be a an isolation valve and should not be used for isolating a process segment. Make sure you select the appropriate valve for the function to be performed.
  • Select materials of construction that will accommodate the media and the process conditions. Take into consideration the parts of the valve that come in contact with process media, such as the valve body, the seat and any other wetted parts. Operating pressure and temperature impact the materials selection for the control valve, too. Conditions surrounding the valve, the ambient atmosphere and specific local conditions that may expose the valve to corrosives should be included in your thinking.
  • Install flow sensors upstream of the control valve. Locating the flow sensor downstream of the control valve exposes it to an unstable flow stream which is caused by turbulent flow in the valve cavity.
  • Establish the degree of control you need for the process and make sure your valve is mechanically capable to perform at that level. Too much dead-band leads to hunting and poor control. Dead-band is roughly defined as the amount of control signal required to affect a change in valve position. It is caused by worn, or loosely fitted mechanical linkages, or as a function of the controller setting. It can also be effected by the tolerances from mechanical sensors, friction inherent in the the valve stems and seats, or from an undersized actuator.
  • Consider stiction. Wikipedia defines it as "the static friction that needs to be overcome to enable relative motion of stationary objects in contact". This can be particularly evident in valves that see limited or no position change. It typically is caused by the valves packing glands, seats or the pressure exerted against the disk or other trim parts. To overcome stiction, additional force needs to be applied by the actuator, which can lead to overshoot and poor control.
  • Tune your loop controller properly. A poorly tuned controller causes overshoot, undershoot and hunting. Make sure your proportional, integral, and derivative values are set.  This is quite easy today using controllers with advanced, precise auto-tuning features.
  • Avoid oversizing control valves. They are frequently sized larger than needed for the flow loop they control. If the control valve is too large, a small percentage of travel or position change could produce an unduly large change in flow, which in turn can make stable control difficult. Unstable control can result in excessive movement and wear on the valve. Try to size a control valve at about 70%-90% of travel.
  • Think about the type of control valve you are using and its inherent flow characteristic. Different types of valves, and their disks, have very different flow characteristics. The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. Globe control valves have linear characteristics which are preferred, while butterfly and gate valves tend to have non-linear flow characteristics, which can cause control problems.  In order to create a linear flow characteristic through a non-linear control valve, manufacturers add specially designed disks or flow orifices which create a desired flow profile.
These are just a few of the more significant criteria to consider when selecting and applying a process control valve. Consider it good practice to discuss your selection and application with a product application expert to confirm your final selection. Combing your process knowledge with their application expertise will provide the best outcome.

Wednesday, July 6, 2016

Wellhead Valves Meet Special Application Challenges

Wellhead in oil and gas industry with equipment and valves
Numerous valves are employed at oil and gas production site
Industrial valves are manufactured in a huge array of configurations to accommodate the specialized needs of a broad range of industrial process applications. The oil and gas industry is but one segment of many throughout the industrial sphere that presents its own set of application specific criteria.

Oil and gas production, essentially pulling raw material from the earth, has unique valve performance challenges. Extreme pressure and abrasive or erosive material are common elements of oil and gas production at the wellhead. The valves also need to tolerate the range of outdoor temperatures at the production site. Safe and reliable operation throughout these and a range of other conditions are part of the design criteria for these valves. Here are some of the specific valve variants and configurations applied in the oil and gas industry at the production wellhead.
swing type check valve for oil and gas production wellhead or manifold
Swing Type Check Valve
Courtesy DHV Indutries
cutaway view of slab gate valve for oil wellhead use
Cutaway view of Slab Gate Valve
Courtesy DHV Industries

  • Slab Gate Valve - Provides metal to metal seal and employs parallel gate and seal with a preloaded spring to assure positive upstream and downstream seal. Full port design allows for pigging.
  • Expanding Gate Valve - A parallel expanding gate seals positively against both seats, which are protected from the flow medium in the open and closed positions.
  • Mud Gate Valve - Designed to provide positive closure under rigorous field conditions with abrasive media. 
  • External Sleeve Adjustable Choke Valve - Designed to regulate production well flow and downstream pressure. Different trim configurations provide appropriate levels of control.
  • Needle Adjustable Choke Valve - Utilizes different trim arrangement than external sleeve type to provide good flow management, abrasion resistance, erosion resistance, and reliable service over a long life with low maintenance requirements.
  • Check Valve - Check valves of various types are utilized throughout practically all fluid flow operations, essentially anywhere that fluid is supposed to flow in only one direction. Oil and gas production presents some special conditions of abrasion, erosion, and pressure that call for special accommodation in design and materials of construction.
There are other specialty valves employed at or near the wellhead, but the key take away here is that oil and gas production generally cannot be accommodated by general purpose valves. I have included a document below that provides additional data and engineering detail for the valves touched upon in this article. Share your oil and gas production challenges with a valve specialist. The combination of your process and production experience with their product expertise will produce effective solutions.