Maintain Inert Gas Purity | Chemical Processing

When dealing with contaminants, the right hardware choices and purging techniques are critical

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Inert gas, by excluding oxygen from vessels, plays a key role in assuring safety and product quality in many chemical processes. It reduces the risk of oxidation that can lead to fire or the deterioration of materials. Keeping the inert gas contaminant-free is essential; however, the mechanics of doing so depend upon how the gas is delivered.

Many large chemical plants consume enormous quantities of inert gas, typically, nitrogen. Demand is so great that a supply pipeline is required. At other plants, where consumption isn't high enough to warrant tapping into the nearest nitrogen pipeline, tank wagon, i.e., truck, delivery from a gas supplier suffices.

Safeguarding pipelined gas

At sites that get nitrogen via pipeline, the first consideration in excluding contaminants is maintaining the integrity of the pipeline supply. To do this, a plant must install valving in its pipe downstream of the pipeline connection. Double block valves and a bleed valve are typically installed, with pressure transducers on either side of the block valves to monitor the nitrogen pressure. A higher nitrogen pressure downstream of the double block and bleed, relative to the upstream nitrogen pressure, indicates contaminant entry. Therefore, if the downstream nitrogen pressure rises above the upstream pressure, the double block valves should close and the bleed valve open, thereby protecting the integrity of the nitrogen pipeline. A signal should be provided to alert operations staff that the block valves have closed and the bleed has opened.

 

Direct from the pipeline

Large consumers of nitrogen generally receive gas by pipeline directly from air separation plants such as this one at Geismar, La.

Source: Air Products and Chemicals

 

The pipeline nitrogen feeds into the site header. At a large chemical complex, each processing unit that requires nitrogen has its own unit header that connects with the site header. Each unit header should begin with a double block and bleed to protect the site header. Pressure transducers should monitor the nitrogen pressure of this second double block and bleed. Again, if the downstream nitrogen pressure exceeds the upstream nitrogen pressure, the block valves should close and the bleed valve open, thus protecting the nitrogen in the site header.

At irregular intervals along the unit header, smaller diameter pipes typically drop from the unit header to utility workstations. These stations provide access to steam, water and nitrogen to plant personnel. Double check valves usually are installed to protect the unit header from contamination originating at a utility workstation. However, the old adage about check valves is "they don't work." So, while check valves offer a sense of security, especially when two are in series, remember this adage at all times. There is always the possibility that one or both check valves will become lodged open, thus increasing the risk of contaminating site nitrogen.

Each service at a utility workstation generally has a block valve in the pipe connecting it to its respective unit header. To access the utility service, plant personnel must connect a hose to the appropriate service and then open the block valve. The hose connections for steam, water and nitrogen differ. This is done for two reasons: first, to enhance safety; and, second, to avoid contamination.

Using different types of connections keeps plant personnel from running a flexible hose from the wrong utility connection to a tank or vessel requiring nitrogen inerting. Preventing misconnections can be particularly critical at a utility workstation that has a compressed air outlet. Running a hose from this outlet to a tank or vessel containing a light hydrocarbon or a hydrocarbon atmosphere can quickly produce a flammable environment, when the original intent had been to inert the tank or vessel with nitrogen.

Inproper hose connections also can lead to contamination. Connecting a utility service, such as nitrogen, to a higher-pressure tank or vessel causes backflow into the service. Having different hose connections at the utility workstation will not eliminate such a contamination event; it does, however, provide a "hesitation point" where thought is required,"i.e., where a potential mistake can be identified and corrected before it can take place.

 

For smaller quantities, truck delivery's the norm

 

Tank wagons deliver nitrogen to plants requiring smaller quantities of the gas. Stored gas usually goes to an evaporator and then to the unit header.

Source: Praxair

 

At the utility workstation, steam uses a boss connection; water a crow's foot connection; and nitrogen an NPT connection. Utility air connects via a pneumatic quick disconnect, while breathing air uses quick connections. It is bad practice to fit different hose connections,"for example, a crow's foot connection and an NPT connection,"to the same bushing. It is potentially deadly to fit a breathing-air quick connection to an NPT connection via a common bushing. Plant personnel fabricate such fittings for various reasons, mainly because they believe this makes whatever task they are undertaking easier. Such devices only increase the likelihood that an unfortunate event will ensue. Whenever these fittings are found, they should be disassembled immediately.

Protecting wagon supplies

For plants that rely on tank wagon delivery of nitrogen, the gas supplier generally provides an on-site nitrogen storage vessel and evaporator. The evaporator directly feeds the unit header.

At such plants, the cost of contaminating the nitrogen storage vessel should be compared to the cost of installing and instrumenting a double block and bleed. For small nitrogen consumers, it is generally cheaper to vent, purge, and refill a contaminated nitrogen storage vessel than it is to install and instrument a double block and bleed.

 

Color coding, valving minimize the risk of contamination

 

A nitrogen line, painted green, at utility workstation at Shell's Geismar, La., complex uses a connection different from that for other services and features two valves to avoid contamination.

 

Two check valves in series just after the evaporator can be used to protect a nitrogen storage vessel and evaporator. Often, however, a single check valve is installed. In most such cases, it is used at the utilities workstation to protect the unit header. Not surprisingly,nitrogen-delivery-system contamination occurs more frequently at plants that rely on single check valves for protection than it does at plants that use two check valves in series in conjunction with a double block and bleed.

Decontaminating gas

If a unit header or site header becomes contaminated, depressure the header and dry it if the contaminant is a liquid. Then pressure and depressure the header three times with nitrogen. This cuts the contaminant level by 99.5 percent. Following the third pressurization/depressurization cycle, allow an amount of nitrogen equivalent to three volumes of the header to flow through it. This will reduce the contaminant further, thereby rendering the header suitable for use again.

Jon H. Worstell is a senior staff chemist for Shell Chemical, Houston. Richard E. Robertson is a senior staff engineer for Shell Chemical, Geismar, La. Jay F. Clark is a senior process engineer for KMTex, Port Arthur, Texas.

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