02252018Sun
Last updateFri, 23 Feb 2018 5pm

i

Achieving Predictable Valve Performance for Safety Applications

Achieving Predictable Valve Performance for Safety Applications

A focus on having the proper specificati...

Putting Servo Valves Back to Work

Putting Servo Valves Back to Work

Industries as varied as paper production...

Corrosion and Fouling: Is There a Solution?

Corrosion and Fouling: Is There a Solution?

According to a 1998 study released by the ...

Valves with All the Trimmings

Valves with All the Trimmings

The term valve trim has been around for ...

SubscribeWNT18

FREE SUBSCRIPTION*

• Print magazine
Digital magazine
• VALVE eNews
Read the latest issue

*to qualified valve professionals in the U.S./Canada

The Weekly Report

New Products

  • ja-news-2
  • ja-news-3

Industry Headlines

Leading Economic Indicators Rose in January

Friday, 23 February 2018  |  Chris Guy

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.0% in January to 108.1, following a 0.6% increase in December, and a 0.4% i...

Readmore

Loading...
Advertisement
i

Web Only

Valve Positioners Offer Improved Control Valve Performance

Valve Positioners Offer Improved Control Valve Performance

Monday, 19 February 2018  |  David Matherly

From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who m...

Readmore

Loading...

Industry Headlines

MRC Global Announces Fourth Quarter 2017 Results

3 DAYS AGO

MRC Global’s sales were $903 million for the fourth quarter of 2017, which was 26% higher than the fourth quarter of 2016 and 6% lower than the third quarter of 2017 due to seasonality. As compared to 2016, all sectors increased driven primarily by midstream and upstream.

MRC Global's fourth qu...

Readmore

Curtiss-Wright Acquires Dresser-Rand Government Business

4 DAYS AGO

Curtiss-Wright has entered into an agreement to acquire the assets that comprise the Dresser-Rand Government Business (Dresser-Rand), a business unit of Siemens Government Technologies, for $212.5 million in cash.

Dresser-Rand designs and manufactures mission-critical, high-speed rotating equipment sol...

Readmore

U.S. Refiners Increasingly Turning to Export Markets

3 DAYS AGO

“U.S. gasoline consumption has leveled off as the stimulus provided by low and falling oil prices between 2014 and 2016 has faded, so refiners are increasingly turning to diesel and customers in emerging markets,” Reuters  reports .

“In geographical terms, the fastest growth in co...

Readmore

$900M Power Plant Planned for Indiana

3 DAYS AGO

Vectren Energy Delivery of Indiana is proposing to install an additional 50 megawatts (MW) of universal solar and build an 800 to 900-MW natural gas-fired generation facility in addition to other critical investments that will significantly change the way the company generates power for the region .

Th...

Readmore

Leading Economic Indicators Rose in January

1 DAY AGO

The Conference Board Leading Economic Index (LEI) for the U.S. increased 1.0% in January to 108.1, following a 0.6% increase in December, and a 0.4% increase in November.

“The U.S. LEI accelerated further in January and continues to point to robust economic growth in the first half of 2018. Whi...

Readmore

IHS Markit Manufacturing Index Rose in February

2 DAYS AGO

U.S. manufacturers reported a strong upturn in business conditions during February, which continued the positive trend seen at the start of 2018. At 55.9, up from 55.5 in January, the seasonally adjusted IHS Markit Flash U.S. Manufacturing Purchasing Managers’ Index (PMI) pointed to the fastes...

Readmore

Advertisement

Common Bellows Failures and Suggestions for Mitigation

vmwnt12_MR_Fig1Figure 1. Galling on the stem due to an oversized valve or operating the PRV too close to set pressure.

While it is an extremely rare event, bellows can and do fail. But bellows failures are often wrongly attributed to the quality of the valve or the bellows while in reality, a more likely scenario is operating conditions or an improperly specified valve that contributed to the failure. Still, whenever a failure occurs, analysis of what happened and why is critical.


THE USE OF BELLOWS

A spring-loaded pressure relief valve (PRV) is a device that reacts based on the amount of static pressure force pushing up on the disc. In normal processing conditions, the valve will remain shut because the upward force on the disc is less than the closing spring force. When the force from the process fluid pushing up and the force of the spring pushing down are at equilibrium, the disc of the valve will begin to lift from the nozzle, and the valve will begin to “simmer.” At this point, a slight increase in process pressure will cause that valve to “pop” open (its set point), thereby relieving the overpressure.

vmwnt12_MR_Fig2Figure 2. Bellows rupture likely because of excessive backpressure.A bellows is typically specified for applications when a spring-loaded PRV will experience backpressure (which can impact the valve’s ability to open at the correct set pressure) or when the internal components of the valve must be isolated from the processing fluid. When selecting the bellows material, consideration of the process material discharging into a common header must be made.

While it is possible for a bellows to fail because of an imperfection in fabrication, failure more commonly can be attributed to the wrong valves being used or operating conditions. Quality control during PRV assembly can prevent a customer from experiencing this type of failure.

Listed below are four scenarios that are common reasons a bellows might fail. Each assumes that a thorough review of the engineering sizing and specifications for a given PRV has been completed since these calculations will aid in diagnosing the problem.


EXCESSIVE BACKPRESSURE

One clue that indicates a valve has been exposed to excessive backpressure is when the bellows has been crushed. There are two types of backpressure in process systems: constant and variable. Variable can be further divided into two subgroups: superimposed and built-up.

Built-up backpressure is defined as the pressure at the outlet of the PRV based on the discharge piping configuration, i.e., pressure that occurs only after the valve has opened. For applications where the flow is compressible, built-up backpressure is based on the piping hydraulics at the accumulation pressure using the maximum actual capacity for the PRV. All too often engineers perform this calculation at the required capacity for the given scenario, not at the device’s actual capacity.

When a bellows failure can be attributed to excessive built-up backpressure, the following options will ­mitigate the problem:

  • Use a bellows with a higher pressure limit.
  • Use a pilot valve balanced against backpressure.
  • Modify the outlet piping by ­making it larger or shortening the length of pipe, thereby ­reducing the effects of built-up backpressure.


OVERSIZED VALVE

While most PRVs are protecting equipment for more than one relief event, the size of the valve is based on the scenario requiring the greatest relieving capacity. An example would be when a PRV is sized for both fire and blocked outlet scenarios. The fire sizing requires significantly greater orifice area than the blocked outlet sizing. However, since the blocked outlet scenario is more common and more likely to occur, then the PRV will be potentially starved for capacity, causing the valve to “chatter” (rapidly opening and closing). Valve chatter, as well as flow instability, could inevitably cause valve damage such as premature fatigue failure of the bellows, as well as galling of guiding surfaces. In our experience, a PRV should not be specified that has an actual orifice area more than 3 to 5 times larger than the required area.

Mitigation strategies for failure in this scenario include:

  • Install multiple PRVs and stagger the set pressure for each of the scenarios. Ensure the small valve is properly sized based on the lowest required capacity relief scenario.
  • Install a modulating pilot-operated relief valve.

  • Latest Post

  • Popular

  • Links

  • Events

Advertisement
BUYERS GUIDE 300x220

Looking for a career in the Valve Industry?

ValveCareers Horiz

To learn more, watch the videos below or visit ValveCareers.com a special initiative of the Valve Manufacturers Association