Pressure and Vacuum Relief Valves

A pressure/vacuum relief valve is part of a complete tank
control system.
Image courtesy Groth Corporation

Storage or processing tanks are a part of almost any industrial liquid processing operation. There are many considerations for the inclusion of a tank in a liquid handling system. One facet, the subject of today's post, is how to accommodate the normal exchange of balance between liquid filled space and non-liquid filled space in a tank as it is filled or drained. Vacuum and pressure relief valves are, in many cases, the best solution for tank venting.

Keeping in mind that some tank involved operations process liquids that must remain isolated or contained from the atmosphere for any of a number of reasons. The tank may be blanketed with nitrogen or another gas. The contained liquid may produce vapor which cannot be freely vented to the atmosphere for regulatory reasons. Whatever the reason, one thing is common among all these applications. The tank must remain a closed system whenever its level is not changing.

When the liquid level in a closed tank changes, the pressure of the air or vapor in the tank will also change. Filling of the tank will increase the pressure, draining will decrease it. It is necessary to offset the change in pressure to avoid tank structural damage or processing difficulties from excessive positive or negative pressure produced by the changing liquid level. Groth Corporation manufactures pressure and vacuum relief valves for processing and storage tanks. Quoting from their installation manual for pressure/vacuum relief valves...

Pressure and/or vacuum relief valves are used on liquid storage tanks and other process vessels or systems to prevent structural damage due to excess internal pressure or vacuum. Storage tanks are pressurized when liquid is pumped in, compressing the existing vapor or when rising temperatures cause increased evaporation or expansion of existing vapor. Conversely, a vacuum condition may be created when pumping out or due to falling temperature. To prevent tank damage, vapor must be allowed into or out of the tank at specified pressure/vacuum conditions. The volume rate of venting depends upon the tank size, volatility of the tank contents, the pumping rates and the temperature.

These valves have specific requirements for installation and maintenance, but their operation is simple. A weighted pallet will move by gravity to a closed position when the liquid level is static. As liquid level changes and the pressure applied to the pallet changes, it will eventually move. It is important to note that each valve is weighted for the customer's specific application, and weight factory applied to the pallet should not be modified or otherwise changed without consulting the valve manufacturer to confirm the new operating parameters.

A pressure/vacuum relief valve is not intended to be a safety valve. It's function is related to the normal operation of the tank. A safety relief valve is an additional component with differing operational requirements. Here is now Groth describes the basic operation of the pressure and vacuum relief valves, with a little editing.

Pressure Relief: As the pressure in the storage tank increases, the vacuum pallet is held shut. When the set pressure is reached, the pressure pallet lifts and relieves tank pressure to the atmosphere (or to a header if it is a pipe away valve). 

Vacuum Relief: As a vacuum is drawn in the storage tank (for example, when fluid is being pumped out), the pressure pallet is held shut. When the vacuum setting is reached, the vacuum pallet lifts and air is drawn into the tank from the atmosphere.

Selecting and specifying a properly sized pressure/vacuum relief valve requires consideration of all aspects of tank operation. Share your project requirements with a product application specialist and leverage your own process knowledge and experience with their product application expertise to develop the best solution.

Combining Rupture Discs With Pressure Relief Valves

Applying a rupture disc in concert with a safety relief
valve can deliver real benefits.
Image courtesy Continental Disc Corporation

Common elements of any pressurized system include safety and pressure relief valves. Their general purpose is to stop system pressure from exceeding a preset value, preventing uncontrolled events that could result in damage to personnel, environment, or assets. Their operating principle and construction are comparatively simple and well understood.

Long term exposure of a relief valve to process media can result in corrosion, material buildup, or other conditions which may shorten the useful life of the valve, or worse, impair its proper operation. This excessive wear will increase the ongoing cost of maintaining or replacing a prematurely worn valve. One other aspect of relief valves can be the reduction in their seal integrity or force as the system pressure approaches the setpoint. This could possibly lead to fugitive emissions, an undesirable condition.

An effective approach to mitigating some of the effects of exposure to the process media is to install a rupture disc upstream of the safety valve inlet. Isolating a relief or safety valve from the process media through the installation of a rupture disc upstream of the valve inlet will eliminate exposure of the costly valve to effects of the media. It is necessary to establish proper rating and selection for the rupture disc to avoid any impairment of the overall operation of the relief valve, but the selection criteria are not complex. A number of benefits can accrue with this concept.

• Rupture disc isolates the valve from the media, allowing application of less costly valves fabricated of non-exotic materials.
• Rupture discs are leak free and bubble tight, eliminating possibility of fugitive emissions from the safety relief valve, especially when system pressure may approach valve setpoint.
• Relief valve inventory can be evaluated for reduction.
• Longer valve life.
• Less downtime.

The additional cost for the rupture disc enhancement can have a reasonable payback period, with all factors considered. In any case, the rupture disc protection makes for a cleaner relief valve installation. The document provided below provides some additional application recommendations and details.

Rupture discs and holders are available in sizes and materials for most applications. Share your ideas with a product specialist, combining your process knowledge with their product application expertise to develop an effective solution.

Combining Rupture Discs and Safety Relief Valves from CTi Controltech

Expanded Product Offering at CTi Controltech

This weight loaded pressure relief valve is one of the many
products from Groth used for safe and efficient tank operations.
Image courtesy Groth Corporation

CTi Controltech has added Groth Corporation to its roster of valve and valve automation products. Groth is a well recognized manufacturer of an array of pressure relief and related products routinely used for tank operations and other processes.

• Pressure/Vacuum Relief Valves
• Pressure Relief Valves
• Vacuum Relief Valves
• Pilot Operated Valves
• Flame and Detonation Arresters
• Emergency Relief Valves
• Waste Gas Burners
• Pressure Regulators

Groth products have been protecting refineries, chemical processing plants and facilities with atmospheric fixed-roof storage tanks for more than 50 years. The new product line complements and expands CTi Controltech's ability to deliver full scale complete solutions to their customers.

A general product guide for the Groth products is included below. More detailed information, as well as product application expertise is available from CTi Controltech. Contact a product application specialist, leveraging your own knowledge and experience with their product application expertise to develop effective solutions to your industrial pressure relief challenges.

Groth Corporation Industrial Product Selection Guide from CTi Controltech

Learning From Catastrophe - Case Study of Heat Exchanger Failure

Shell and tube heat exchangers at industrial plant

Industrial accidents, whether minor or catastrophic, can serve as sources of learning when analyzed and studied. Operators, owners, and technicians involved with industrial chemical operations have a degree of moral, ethical, and legal responsibility to conduct work in a reasonably and predictably safe manner without endangering personnel, property, or the environment. Part of a diligent safety culture should include reviewing industrial accidents at other facilities. There is much to learn from these unfortunate events, even when they happen in an industry that may seem somewhat removed from our own.

The U.S. Chemical Safety Board, or CSB, is an independent federal agency that investigates industrial chemical accidents. Below, find one of their video reenactments and analysis of an explosion that occurred at a Louisiana chemical processing plant in 2013. A portion of the reenactment shows how a few seemingly innocuous oversights can combine with other unrecognized conditions that result in a major conflagration.

Check out the video and sharpen your sense of awareness for potential trouble spots in your own operation.

Pressure Relief Valves

Brass Safety Valve - One of many types of safety valves

Danger and hazards are an integral part of industrial processes. The mitigation of these dangers and hazards, as well as reducing the probability of their occurrence, is the primary charge of industrial process engineering. Every product intended for use in a process control setting has safety and protection included in its design criteria. Pressure relief valves fall in that category of products designed and intended solely for safety purposes.

Manufacturers of what most generally refer to as pressure relief valves break the genre down into two distinct groups, relief valves and safety valves. One manufacturer, Anderson Greenwood (a Pentair brand), distinguishes the two valve types in their "Pentair Pressure Relief Valve Engineering Handbook"...

Relief Valve: A pressure relief valve characterized by gradual opening or closing generally proportional to the increase or decrease in pressure. It is normally used for incompressible fluids.

Safety Valve: A pressure relief valve characterized by rapid opening or closing and normally used to relieve compressible fluids.

The difference between the two valve types is found in their response to an excessive pressure condition. The relief valve, according to the definition, responds proportionally to the pressure increase, whereas the safety valve provides a non-proportional rapid response. Note also that the relief valve is generally intended for use with liquids (incompressible) and safety valves are commonly applied to compressible fluids, which would include steam and air.

Pressure relief valves are found anywhere pressure is contained, be it a piping system, vessel, even a household pressure cooker. The purpose of the relief or safety valve is to protect a pressurized system or vessel, should the system pressure exceed the maximum allowable working pressure. Simply put, keep it from breaking apart.

Because of the potentially catastrophic nature of a pressurized system failure, there is a high level of scrutiny, regulation, and testing focused on pressure relief and safety valves. The proper sizing and selection of the valves is also critical to providing proper function.

More detailed information, product data, and application assistance is available from the specialists at CTI Controltech.