Perspectives: Field Notes

Get Your Head Around Shutoff Head

Specifying control valves that may need to provide tight cutoff requires care

By Dirk Willard, Contributing Editor

A data sheet for a reactor flow-control valve called for a “shutoff head.” Given that this control valve handled ethylene oxide (EO), a highly volatile and reactive compound, the requirement gave me pause. Addressing it involved more than defining the pump deadhead pressure. Here’re some thoughts to consider.

First, it’s important to understand that a control valve isn’t a shutoff valve. A control valve must provide precise adjustments to flow. Crunching down on the plug to block flow can cause damage. The actuator and linkage of a control valve aren’t designed for tight shutoff; a shutoff valve’s actuator is. If you want to prevent leakage, install a shutoff valve.

Unfortunately, no standards define control-valve shutoff head. A few describe leakage through the body of a valve; this leakage is categorized poorly as Class I-VI in ANSI/FCI 70-2, “Control Valve Seat Leakage.” Valves also leak from their stems. For stem leakage, refer to ISO-15848, “Industrial Valves — Measurement, Test and Qualification Procedures for Emissions – Part 1...” This specification is best because it recommends helium as the test medium. API-622, “Type Testing of Process Valve Packing for Fugitive Emissions,” also describes stem leakage but for methane.

However, we’re concerned about defining the shutoff head for a control valve specification, not leakage. For this, understanding the limits of valve body type is important. Obviously, butterfly valves won’t provide tight shutoff — although I have seen them used where leaking is acceptable, such as where thermal expansion is allowed. Be aware that thermal expansion could cause the valve seat to stretch or dislodge. In this case, the valve itself limits the shutoff head. Butterfly valves are available with shutoff heads of 175 psi and up, but, in practical terms, the limit is more like 50 psi or less. Diaphragm and other soft-seat valves have the same problem. Gate valves, with their exposed seats and metal-on-metal sealing surfaces, are a bad choice. Globe valves, while often most effective for control, are the worst option. Ball and plug valves probably offer the best choice for shutoff.

Lack of mechanical strength is a common problem with soft seats but less of an issue with hard, metal-on-metal seats. Soft seats are best for sealing against streams containing solids, such as slurries. Hard surfaces, polished smooth, work best at high temperatures, above 250°F, where polymers begin to break down or flow; solids quickly ruin polished surfaces. Fluoropolymers can extend the range of soft seats to 350°F. However, hard plastics retain memory and can be scratched just like metal; memory retention gets worse above 200°F. Partially-fluorinated polymers generally outperform polytetrafluoroethylene (PTFE) in fabricated liners but PTFE serves well as sheets. Ceramics offer a useful alternative to metal in erosive environments but not without sacrificing tight shutoff.

Enough about the peripherals, let’s get down to specifications. Several scenarios can define shutoff head: 1) flow provided by a mover, e.g., a pump or compressor; 2) pressure isolation; 3) temperature control; and 4) safety, i.e., prevention of a chemical reaction. In the first case, use the deadhead pressure of the mover. (Deadhead is the extreme left corner on a pump curve.) For a pump, pick the deadhead for the largest impeller possible. For temperature control, use the regulator steam pressure — unless a significant risk is present and the steam is stepped down, in which case use the original pressure. Safety is a tricky subject.

If a chemical such as EO is involved, safety trumps a pump curve. EO leaking past the isolation valves to the reactor could lead to a release or worse. In this situation, as in the others requiring a tradeoff between control and isolation, don’t rely on a control valve for shutoff: put automatic valves upstream and downstream of the control valve for isolation. Install pressure switches and pressure transmitters to monitor thermal expansion and use thermal relief valves to blow expanding liquids back to storage. Both switches and transmitters are valuable because transmitters are limited to <100-ms response time and switches don’t trend.

Lastly, let’s look at actuators. If a control valve is to function as a reliable shutoff valve, a positioner is an absolute necessity. Ideally, use one with a 0–100% position indicator. Specify an open-and-closed position indicator for all automatic valves used for shutoff. Take care in selecting the actuator air pressure and supply; evaluate the pressure reliability before selection; plan for 1 scfm of air per control valve or automatic valve that’s 6 in. or less in size.


dirk.jpgDIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at dwillard@putman.net

More from this perspective...

Title

Don't Devalue Isometrics

They sometimes can tell you a lot more than P&IDs

10/19/2011

Don't Blame Regulations

Environmental and safety mandates don't harm U.S. economic growth.

11/05/2012

Don't Add Pressure to Relief Valve Surveys

Consider the time and cost necessary to do a proper analysis.

04/18/2012

Do Your Level Best

Level control with dP transmitters appears simple but really isn't.

11/29/2010

Diagram Process Design History: Don't Trip Up on Line Lists

Resolve discrepancies and other problems early.

05/06/2013

Defuse Dust Dangers

Carefully consider and then counter risks of fire and explosion

08/27/2013

Deftly Deal with Management of Change

Consider some proven tactics to survive this grueling process.

11/20/2008

Correctly Manage Change

How you begin is at least as important as your eventual goal

10/10/2008

Contracts Don't Come In On Their Own

Your ability to sell may save you and your company.

08/02/2012

Consider the Consequences of Chemistry

Reactions can cause a wide variety of non-process problems.

03/06/2013

Choosing the right contract

A contract can be a stone in your shoe. Knowing which type of contract to choose can make your job easier. Chemical Processing's Senior Editor Dirk Willard provides tips on various types of contracts.

09/20/2006

Choose Your Next Position Wisely

Finding your dream job takes a bit of soul searching and answering some tough questions.

01/20/2009

Choose the right fluid for your pressure test

Compressed gas often makes the most sense for checking a process, according to Senior Editor Dirk Willard, in this month's Field Notes column.

04/12/2007

Check out before you start up

In his monthly Field Notes column, Senior Editor Dirk Willard reveals that regardless of the preparation, check-out tests always bring out some surprises.

02/28/2006

Carefully Commission Hydrogen Pipe

Safety depends upon scrupulously performing a variety of inspections and tests

06/11/2014

Build reliability in during design

Instrumentation that promises to improve reliability may not always be what they seem. You have to consider whether a site can properly maintain sophisticated instruments.

12/19/2007

Brain Drain Brings Big Headaches

Inexperience and a tough production schedule can be a lethal combination

02/15/2009

Avoid Surprises During Pump Startup

A well-thought-out approach can eliminate potential problems.

08/26/2010

Avoid Project Management Mistakes

A number of common mistakes can sabotage success

11/20/2013

Avoid Problems with Random Packing

Careful internals' selection is crucial for absorbers and scrubbers.

11/28/2012