Understanding Differential Pressure Measurement: Differential Pressure Gauge Example


This video (courtesy of Ashcroft) does an outstanding job illustrating the concepts of differential pressure and flow measurement using the differential pressure method.

Engineered restriction devices are often inserted into a closed pipe system to create a differential pressure for the purposes of measuring fluid flow rate. These restrictions can come in the form of an orifice plates, Venturi, wedge, and other designs.

To measure the differential pressure, taps must be installed on both sides of the plate.  The upstream side will always produce the greater pressure, and is referred to as the high side. Conversely, the downstream pressure will always be the lesser value, due to the obstruction.

A differential pressure gauge's range is based on the maximum difference that can be expected as a result of the restriction. The gauge's dial will display the differential pressure in units of pressure measurement, like psi or bar.  By applying the linear square root relationship between flow rate and pressure, the gauge style can be scaled in a specified rate of flow, such as gallons per minute. A dual scale dial can also be created to display both the flow rate and the differential pressure.

Another important consideration is the maximum line pressure, also referred to as the static pressure. The higher the static pressure, the more robust the gauge must be to contain it. That's why it's crucial to ensure that the gauge carries a static pressure rating that exceeds the highest pressure in the line.

For more information about differential pressure gauges, transmitters, and flow measurement, contact Mead O'Brien at (800) 892-2769 or visit their web site at https://meadobrien.com.

ValvTechnologies RiTech® Coating an Excellent Alternative for Applications Where Stellite Disbonding a Concern

ValvTechnologies
Stellite is a trademarked name of Kennametal Inc. describing a range of cobalt-chromium alloys designed for wear resistance. Commonly used on severe service valves, Stellite alloys operate at high temperatures (600 – 1112° F), can be polished to excellent levels of surface finish producing low friction coefficients and in-turn providing good sliding wear. In high-temperature, high-pressure steam applications, however, there are reported issues of Stellite delamination when valves operate at Stellite's upper operating temperature range.

ValvTechnologies RiTech31
Table of ValvTechnologies RiTech Coatings
(Click for larger view)


ValvTechnologies, a manufacturer of severe service valves, offers their RiTech® coatings and process as a better alternative to Stellite for these applications.

ValvTechnologies' RiTech® is a high-velocity oxygen fuel (HVOF), hot, high-velocity, gas jet coating process. RiTech® 31 is an alloy that maintains its hardness at high temperatures and is self-repairing in operation.

The article below, written by the editors of the Combined Cycle Journal and distributed by ValvTechnologies, explains the reported Stellite delamination problems as well as a RiTech® 31 user experience.

For more  information about ValvTechnologies valves and RiTech® 31, contact Mead O'Brien by calling (800) 892-2769 or by visiting https://meadobrien.com.


Coating Critical Steam-valve Parts with Chrome Carbide Avoids Stellite Delamination Issue

Stellite liberation from large valves installed in main and hot reheat (HRH) steam systems serving F-class combined cycles, considered a major industry problem 10 years ago, has been eliminated by substituting chrome carbide as the hard-facing material for critical valve parts.

The editors first learned of stellite delamination at the 2009 7F Users Group Conference where the liberated material from a 20-in. HRH block valve was displayed. The industry had been made aware of stellite liberation by GE, which issued Technical Information Letter 1626 about three months ahead of the 7F meeting. It advised steam-turbine owners to check the condition of the stellite inlay sections used in fabricating seats for the OEM’s combined stop and control valves.

Revision 1 of that TIL, published at the end of 2010, recommended a “one-time seat stellite inlay UT inspection during valve installation or the next planned maintenance inspection”—this to identify any lack of bonding between the inlay and base metal on units with fewer than 50 starts.

Disbonding of stellite associated with combined-cycle plants has occurred primarily in parallel-slide gate valves and non-return globe valves. Hardfacing has been liberated from valve seats, guide rails, and discs. Tight shutoff of valves has been compromised in some cases.

Many incidents of stellite liberation were reported. To illustrate: CFM/VR-TESCO LLC (formerly Continental Field Machining), a leading valve services company said that in 2011 and 2012 it repaired 50 valves manufactured from F91 (forged body) or C12A (cast body) and ranging in size from 12 to 24 in. More than half of these jobs involved stellite liberation.

These repair projects were split roughly 50/50 between valves within the Code (ASME Boiler & Pressure Vessel Code) boundary and those that were part of the boiler external piping. Repairs on the former were performed according to guidelines presented in Section I of the Code and in the National Board Inspection Code; those outside the Code boundary were performed according to ASME B31.1.

There hasn’t been much discussion on stellite disbonding the last few years—at least at meetings attended by the editors, which include the Combined Cycle Users Group, Steam Turbine Users Group, and HRSG Forum with Bob Anderson.

However, mention was made by one owner/operator regarding the successful use of ValvTechnologies Inc.’s IsoTech® parallel-slide gate valves on his company’s HRSGs in eliminating the need for stellite. According to the manufacturer, critical parts for its severe-service valves, used where steam temperatures exceed 1000F, are provided with its RiTech® 31 coating.

This chrome carbide refractory coating is much harder than Stellite 6 (68-72 RC versus 34-38 RC). It is applied in state-of-the-art HVOF (high-velocity oxygen fuel) spray booths using a proprietary compressive spray technique to achieve high bond strength. Applications extend up to ASME/ANSI Class 4500 at 1800F for valves up to 36 in.

The chrome carbide hard-coated web guide ensures the discs are kept parallel through the entire valve stroke. As the valve is cycled under differential pressure, the hard surfaces reportedly burnish and polish each other, avoiding the scratching and galling cited by some others.

The user sharing his experience with the ValvTechnologies product said their parallel slide gate valves have been operating on four or five of his company’s HRSGs for three years or so and the only hiccup was a stem-packing leak on one valve which was quickly corrected. This testifies to the vendor’s claim that RiTech 31 hard-coating technology is impervious to the effects of high-temperature cycling typically experienced today in combined-cycle main-steam isolation and HRH applications. The company guarantees coating integrity for 10 years or 10,000 cycles—whichever comes first.

Finally, the user mentioned that a representative of the manufacturer annually visits each plant where ValvTechnologies valves are installed to verify that they continue to meet expectations.

Courtesy of Combined Cycle Journal. Combined Cycle Journal is the independent voice of the gas-turbine-based generation sector of America’s electricity industry.

Pneumatic Valve Actuators

scotch yoke actuator
Actuated valve with pneumatic
scotch yoke actuator (Metso Neles)
Pneumatic valve actuators are used in extreme conditions in many industries such as oil and gas, chemical, water and wastewater, bulk storage, pulp & paper, and power generation. These devices are used in a multitude of valve control processes for regulation (or cessation) of flow, and / or controlling pressure and level.  Due to their reliability and simplicity, pneumatic actuators are one of the most popular types of actuators used in industry today.

Pneumatic valve actuators work by conversion of air pressure into motion. The device applies a force of air to a diaphragm, rotary vane, or piston that is attached to the actuator shaft, which is then mechanically connected to the stem of the valve or damper. Depending on the type, pneumatic actuators produce either linear or rotary motion. 

ACTUATOR ACTION - SPRING RETURN OR DOUBLE ACTING

Spring Return — Pneumatic actuators with spring return design have air supplied from one side. The spring on the opposite side is responsible for the motion. With this design, air compression moves the opens or shuts the valves while the spring is responsible for the opposite motion. 

Diaphragm actuator
Diaphragm actuator
(Metso Neles)
Double Acting  — Double acting actuators have air fed on both sides of a piston. The pressure on one side is higher as compared to the other that results in the required in movement. Air is used to open and close the valves.  

PNEUMATIC ACTUATOR DESIGNS

Diaphragm Actuators — Diaphragm actuators work by applying pressure to a thin membrane or diaphragm.  

Piston Actuators — Piston actuators apply compress air to a piston that is within a cylinder. Air is fed into a chamber that moves the piston in one direction. The piston moves in the opposite direction when air pressure is removed (spring assisted) or directed to the other side (double acting). 

Rack and Pinion — Rack and pinion actuators produce rotation by applying pressure to pistons with gears that turn a pinion gear. Rack and pinion actuators can be spring return or double acting. They are valued because of their compact size and versatility.
Rack and pinion actuator
Rack and pinion actuator
(Metso Jamesbury)

Scotch Yoke — A scotch-yoke actuator contains a piston, yoke, connecting shaft, and rotary pin. They can be direct acting or spring return. They are capable of providing very high torque outputs and are generally used on larger valves. Scotch yoke actuators can be powered by air or process gas.

Rotary Vane —Vane actuators use a mechanical vane, connected to a shaft, that separates a circular shaped body in two "clamshell" halves. The vane moves in response to the differential pressure inside the actuator body, turning the shaft clockwise or counter-clockwise in response to the pressure differential. External springs units are available for spring return models.
scotch yoke actuator
Scotch yoke actuator (Metso Neles)

BENEFITS OF PNEUMATIC ACTUATORS

The use of compressed air (typically found in all industrial facilities) as the power source is the prime advantage for the use of pneumatic actuators. Additionally, pneumatic actuators have an advantage in suitability for different environments and can be used in extremes temperatures. They are preferred over electrical actuators in explosive, flammable and other hazardous areas because they do not require electricity (a possible ignition source) to operate. They do not create electrical fields or electrical noise since there is no electrical motor. Pneumatic valve actuators are faster opening and closing compared to their electric counterparts. Finally, they are low cost, lightweight, durable, require little maintenance (depending on quality) and there are a myriad of positioning controls, speed controls, and communications devices available for tailoring the actuator to the application.

DRAWBACKS OF PNEUMATIC ACTUATORS

While compressed air is the main reason for using pneumatic actuators, it can also be considered a drawback. For instance, pneumatic actuators can perform poorly when the air supply source is located at a distance, resulting in lag and slow response. Another drawback of pneumatic actuators is the additional cost for the compressed air system due to the requirement of dust filters and moisture removing dryers. These are required to ensure clean air is fed into the system.

APPLYING PNEUMATIC ACTUATORS

There are many aspects to the proper, safe, and efficient application of pneumatic actuators to valves and dampers. The sizing the power (torque) output being paramount. All valves and dampers have unique torque requirements. You must consider a threshold force for opening (breakaway), as the valve continues to move to its open or closed position, and then for seating. Matching the actuators to the valve type, and operating conditions is critical. Published torque curves must be reviewed and understood. Too little torque and the valve will not respond. Too much torque increases cost and can damage the valve. Spring return adds to this complexity. Considering all this, it is strongly suggested you always discuss any valve actuation requirement with an experience applications expert. They will ensure the proper, safe, and cost effective mating of pneumatic actuator to valve or damper.

Humidification Design Checklist: Getting It Right The First Time

It’s not enough for a humidification system to simply add moisture to dry air. Control of relative humidity is essential – even critical – in some applications. Yet, there are a variety of factors and individuals that converge making it complex and challenging to design proper humidification systems.

This white paper, courtesy of Armstrong International, provides important information on all aspects of humidification.

Topics Discussed:
  • Direct Steam Injection (conventional separator type)
  • Direct Steam Injection (short absorption panel type)
  • Steam-to-Steam Humidifiers
  • Electric (steam generating) Humidifiers
  • Gas Fired Humidifiers
  • Fogging Systems (Compressed Air and Water)
  • Fogging Systems (High Pressure Atomizer)


White Paper "Valve Actuation: The When, How and Why of Actuator Selection"

Valve automation dramatically improves control, precision and efficiency. While these advantages
must be weighed against higher capital and operating costs, actuators are frequently found to be a cost-effective investment for most oil and gas applications. This white paper, courtesy of Flowserve, is excellently written and thorough.

TOPICS DISCUSSED
  • Is automation right for your application?
  • The actuator hierarchy
  • Electric actuation
  • Pneumatic actuation
  • Hydraulic actuation
  • Specialized actuators for pipeline applications
  • Gas-powered actuators
  • Electrohydraulic (EH) actuators
  • Future developments


Metso Neles High Performance Butterfly Valves for Industry


BW Series Butterfly Valve For High Pressure Applications

Neles BW series metal seated triple eccentric disc valve is designed for both control and tight shut-off in severe service. Multiple seat options makes it well suited for the refining, power, petrochemical and chemical industries. The BW series provides extended operational life in high cycle, high temperature and abrasive applications.

High Cycling Butterfly Valve, Series BO

Metso Neles BO series butterfly valve is designed for Vacuum Pressure Swing Adsorption plants (VPSA). It is a wafer type, soft seated, high performance, single eccentric disc valve. The valve provides maximal endurance in fast cycling. Additionally, it provides long lasting tight shut off with excellent flow characteristics and high capacity while maintaining the internal tightness even up to 2 million cycles.

High Performance Triple Eccentric Disc Valves, Series L6

Neldisc series L6 triple eccentric disc valve operate both in control and shut-off applications, with close to equal percentage characteristics and superior tightness.

High Performance Triple Eccentric Series L1 and L2

Neldisc series L1 and L2 triple eccentric disc valves operate both in control and shut-off applications, with close to equal percentage characteristics and superior tightness.

High Performance Triple Eccentric Series L12

Neldisc series L12 triple eccentric disc valve operate both in control and shut-off applications, with close to equal percentage characteristics and superior tightness.

High Performance Triple Eccentric Series LW & LG 

Neldisc series LW and LG triple eccentric disc valves operate both in control and shut-off applications, with close to equal percentage characteristics and superior tightness.


For more information, contact Mead O'Brien:
https://meadobrien.com
(800) 892-2769