06302016Thu
Last updateWed, 29 Jun 2016 5pm

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Fugitive Emissions—Issues and Opportunities

Fugitive Emissions—Issues and Opportunities

Fugitive emissions, ppm, consent decrees...

Problem-Driven Innovation

Problem-Driven Innovation

Developing Alternative Technology to Imp...

Valve Repair Takes Center Stage in Houston

Valve Repair Takes Center Stage in Houston

Attendees gathered June 2-3 in Houston t...

What Internal Best Practices Can Do for Valve Selection

What Internal Best Practices Can Do for Valve Selection

As time goes by, technology moves forwar...

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Industry Headlines

Emerson Selected to Upgrade SE Virginia Wastewater Utility

12 HOURS AGO

Emerson Process Management is replacing an outdated supervisory control and data acquisition (SCADA) system as well as remote terminal units (RTUs) used throughout Hampton Roads Sanitation District’s (HRSD) wastewater collection system. Currently, HRSD serves about 1.7 million people over an are...

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MSS Publishes Revised Standard for High Pressure Knife Gate Valves

1 DAY AGO

The Manufacturers Standardization Society (MSS) announces that recently revised Standard Practice, SP-135-2016, High Pressure Knife Gate Valves, has been approved by the American National Standards Institute (ANSI) as an American National Standard (ANS).

The first edition of MSS SP-135 was published in...

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U.S. Crude Storage Capacity Utilization Rises Even as Capacity Grows

10 HOURS AGO

Weekly U.S. commercial crude oil inventories have increased by more than 71 million barrels (15%) since the end of September, pushing crude oil storage capacity utilization to a near record high of 73% for the week ending June 10.

The U.S. Energy Information Administration (EIA) measures crude oil stor...

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Oil Sands Production Forecast to Grow by Nearly 1M bpd

1 DAY AGO

IHS, in its outlook for Canadian oil sands production through 2025, expects continued growth through the period. IHS anticipates a new phase driven primarily by the expansion of existing facilities with more attractive economics.

Oil sands production is forecast to grow by nearly one million barrels pe...

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Trump Comes Out Against Trade, Draws Pushback

12 HOURS AGO

GOP presidential candidate Donald Trump, speaking Tuesday near Pittsburgh, expressed fierce opposition to free-trade agreements and criticized Hillary Clinton for supporting them. Trump blamed Clinton and her husband for the loss of millions of manufacturing jobs and threatened to pull the U.S. out ...

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Consumer Confidence Climbs to 8-Month High

1 DAY AGO

The Conference Board Consumer Confidence Index, which had decreased in May, improved in June. The Index now stands at 98.0 , up from 92.4 in May. Those stating business conditions are “good” increased slightly from 26.1% to 26.9%, while those saying business conditions are “bad&rdquo...

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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.

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