Monday, May 22, 2017

Introduction to Valve Parts or Components

cutaway view forged steel gate valve
Cutaway view of a forged steel gate valve
Courtesy Crane-ChemPharmaEnergy
Although there are many different classifications of valves specific to their respective functions, there are standard parts or components of valves you may find regardless of the classification. They are the valve body, bonnet, trim, seat, stem, actuator, and packing.

The Valve Body is the primary boundary of a pressure valve which serves as the framework for the entire valve’s assembly. The body resists fluid pressure loads from connected inlet and outlet piping; the piping is connected through threaded, bolted, or welded joints.

The Valve Bonnet is the opening of the Valve Body’s cover. Bonnets can vary in design and model, is built using the same material as the Valve Body, and is also connected to the entire assembly through threaded, bolted, or welded joints.

The Valve Trim collectively refers to all the replaceable parts in a valve, e.g. the disk, seat, stem, and sleeves––all which guide the stem as well.

The Valve Disk allows the passage or stoppage of flow. Disks provide reliable wear properties and differ in what they look like per valve type. For example, in the case of a ball valve, the disk is called a ball, whereas for a plug valve it is a plug.

The Valve Seat(s) or it’s seal rings provide surface seating for the disk. For example, a globe valve requires only one seat and this seat forms a seal with the disk to stop flow.

The Valve Stem provides the proper position which will allow the opening and closing movement of the Valve Disk. Therefore, it is connected to the Valve Disk on one end and the Valve Hand Wheel or the Valve Actuator on the other.

The Valve Yoke is the final piece in the valve’s assembly; the Yoke connects the Valve Bonnet with the actuating mechanism. The Valve Stem passes through the top of the Yoke which holds the Yoke or stem nut.

There are countless variants of valve designs, sizes, and configurations. These basic parts will be found on most, but the particular form and arrangement of the part may provide an advantage when employed for a particular application. Share your industrial process valve requirements and challenges with a valve specialist. Combine your own process knowledge and experience with their product application expertise to develop an effective solution.

Monday, May 15, 2017

Wireless Communications in Industrial Process Control

symbolic wireless communications or transmission tower antenna
Industrial wireless communications for process control
Electrical cables, for many years, were the only option for connecting measurement devices with their companion control and monitoring gear. While wired connections are still in widespread use, probably still the predominant connection method, wireless communication technology offers a range of advantages in connecting process measurement instruments and their controls.

Bandwidth, in wireless communication and modem data transmission terminology, is the analog range of the radio spectrum’s frequencies, or wavelengths, used to transmit a signal between transmitter and receiver. Jurisdictional agencies throughout the world regulate the use of bandwidth and assign ranges for use by public and private organizations.

Before there can be an appreciation of radio transmissions, there first must be an understanding on the transmission medium. Radio transmissions run on the UHF radio spectrum, or the Ultra high frequency spectrum, named by the International Telecommunication Union; the spectrum runs from 300 megahertz (MHz) to 3 gigahertz (GHz).

There are two prominent frequencies utilized for industrial wireless communications in the US: 900 MHz and 2.4 GHz. Within the allocated bandwidth, there are numerous individual channels that can be used for applications.

Each of the available bandwidths has its own transmission characteristics which may make it advantageous for a particular application. Amateur radio stations operate in the 900 MHz range because attenuation of the transmission signal is less than at higher frequencies. Higher frequencies have greater theoretical transmission range, but can be impaired by smaller sized objects in the transmission path because of their shorter wavelength.

Due to a number of practical application factors, 2.4 GHz technology is predominate in consumer and many industrial applications; one of the main reasons is the sheer amount of concurrent signals in the designated bandwidth. The 2.4 GHz band can accommodate more concurrent users or devices. Both frequency ranges have useful application in automation and process control, enabling effective connections between devices over distances from several feet to thousands of miles.

Thursday, May 11, 2017

Rack and Pinion Actuator - Double Acting vs. Single Acting

pneumatic valve actuator
Pneumatic rack and pinion valve actuator
Courtesy Emerson - Hytork
Automating industrial valve operation requires numerous considerations in selecting the correct power source, drive type, torque range, and much more. The widest range of possible operation conditions should be anticipated and accommodated by the actuator selection to assure safe and effective valve operation under normal and adverse conditions.

The use of compressed air or gas as the energy source for valve positioning has been in use for many years and remains popular to this day. Among the perceived advantages of this energy source are the ability to store it in pressurized vessels for emergency short term use and the absence of any potential ignition source, as may be the case with electric powered actuators.

A rack and pinion valve actuator delivers a linear torque output throughout its full range of travel. The movement of a piston causes movement of the rack. The rack is toothed, and drives the pinion, converting linear movement of the rack into rotational movement of the pinion. The pinion is connected to the valve shaft, providing re-positioning of the valve. Adjustable stops, part of the actuator, limit the travel of the valve trim.

spring return and double acting valve actuator diagrams
Double acting pneumatic rack and pinion actuator (left) on its inward stroke. Spring return actuator (right) on its
outward or air powered stroke  (Illustrations courtesy of Emerson - Hytork) 

There are two common configurations of rack and pneumatic pinion actuators. A double acting actuator has provisions for delivering or exhausting air from both sides of the piston. Small control valves coordinate the delivery and removal of pressurized air or gas to drive the pistons inward or outward, producing torque in a clockwise or counterclockwise direction. Its operation could also be described as "air to open, air to close".

The single acting version of the pneumatic rack and pinion actuator provides air driven movement in only one direction. In this case, reversing the direction of travel is accomplished with a spring installed within the chamber on one side of the pistons. The spring powered movement provides a fail safe positioning of the valve in the case of control air pressure loss. This actuator provides "air to open, spring to close" operation, although, in some cases the fail safe position can be changed.

This is the simple version. Share your process control challenges with a valve expert, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Tuesday, May 2, 2017

Steam Traps

high pressure float type steam trap cutaway view
Cutaway view of high pressure float type steam trap
Courtesy Spirax Sarco
Steam is widely used throughout industrial, commercial, and institutional facilities and a means of transferring heat energy, as well as a wide array of other applications. Steam generation cost is a substantial line item on almost any balance sheet, so deriving the most efficient level of operation from a steam system pays tangible dividends.

Utilizing the heat content of steam, in a closed system, results in the production of condensate. Condensate is hot liquid water which can be returned to the boiler and re-vaporized. Managing the separation of the liquid condensate from the process steam and sending it to the lower pressure condensate return line is the function of a steam trap. A steam trap filters out condensate (condensed steam) via an automatic valve. The trap also removes air without letting process steam escape. By filtering out the condensate and not the steam, steam waste is minimized. Steam traps generally are self-contained automatic devices. Since steam based heating processes generally rely on latent heat transfer for rapid and efficient operation, it is necessary to continually collect and transfer condensate from the steam containing portion of the system. The condensate will reduce heat exchanger performance if allowed to accumulate.

Historically, there have been three main types of steam traps: mechanical traps, thermostatic traps, and thermodynamic traps. Most commonly used mechanisms rely on differences in temperature, specific gravity, and pressure. The mechanical trap was originally developed as a bucket trap, which was a rather large trap where a bucket floated up or down to open and close a valve. Bucket traps with a lever, which face downward – also known as ‘closed bucket’ traps – are still used today as a float type trap. Processes requiring large capacities for discharge still currently use the bucket type or float type trap, with long services lives. In the modern version of a free float trap, the condensate is continuously discharged while the valve opening is constantly controlled by the amount of buoyant force acting upon a tightly sealed float.

Thermostatic traps are a smaller, more compact design. Using a temperature sensing mechanism, and operable by mechanisms like bellows or bimetal rings, these thermostatic traps have a slower response. Processes relying on rapid condensate discharge most likely will not use thermostatic traps. An example of a trap used in the process industry today is a bimetal temperature control trap. The trap includes steam tracers and will discharge when a certain condensate temperature is reached.

The core limitation of thermostatic steam traps – the slow response time – has been addressed via the development of the thermodynamic steam trap. The thermodynamic trap operates on the expansion and contraction of an encapsulated liquid. This version of steam trap allows for the smallest amount of condensate accumulation. Early models resulted in unacceptable levels of steam loss. As a result, the commonly used disc type trap was developed for mainstream use. The disc type is compact, versatile, and relatively affordable in terms of installation costs. In the modern disc type, pressure fluctuations in the chamber above the valve result in the valve’s opening and closing. Though in use for many years, development and refinement continues on steam traps, bringing ever better performance to this ubiquitous steam specialty.

Share your challenges with a steam system specialist, combining your own process and facilities knowledge and experience with their product application expertise to develop effective solutions.