Natural Gas Odorization Unit

• Natural gas by nature is odourless which makes it impossible to detect when leakage occurs.
• In order to impart smell, odorant is added to the gas before it enters the medium pressure distribution system.
• The lower explosive limit for natural gas in a mixture with air is approximately 5%. Therefore the presence of natural gas must be readily detectable at one fifth of the lower explosive limit or 1% gas in the presence of air.
• Generally odorant systems are installed at CGS, where custody transfer of gas takes place.
• Modern gas odorants can be divided into sulphur based & sulphur free odorants. Sulphur based odorants can be further subdivided in alkyl mercaptans, alkyl sulphide & cyclic sulphide. New sulphur free acrylades based odorants are more environment friendly due to zero sulphur dioxide emission after gas combustion.
• Ideally odorants have strong odour and must remain traceable as long as fault of technical equipment is detected.
• Odorant should be chemically stable and must have high vapour pressure in order to avoid condensation at operating pressure.
• The odorant should not react with gas componentss, piping material, rust, etc.
• Odorants must not contain water and must not be diluted with water due to possible subsequent corrosion of the equipment.
• After odorization, gas at a concentration as low as 1% by volume, should be detectable.
• While selecting the suitable odorant system, several factors are taken into consideration such as location of system, gas flow volume, type of measuring equipment, injection rate, type of injection system & age of pipeline.
• According to the PNGRB guidelines, a safe distance of 1.5 m around the odorizing system is to be maintained for ease of maintenance and personnel movement.
• The odorizing system is usually located in a separate area at the CGS. It is also mandatory to keep a provision of odorant absorbers like activated carbon saw dust, dry sand & odorant neutralizer like sodium hypochlorite for handling the spillage of odorant.
• As a safety provision, use of personal protective equipment like face shield, mask, rubber hand gloves, gumboot, safety goggles etc. is mandatory while handling odorant spillage.
• In most CGS set-ups a combination of mercaptans is used as odorizer. There are various types of odorants which are available; however most of the companies are using ethyl mercaptan. The types of odorants available are as follows;
• Tetrahydrothiophene (TMT), Dimethyl Sulphide (DMS), Diethyl Sulphide (DES), Methylethyl Sulphide (MES), Ehtyl Mercaptan (EM), Sec butyl Mercapton (SBM), Tertiary Butyl Mercapton (TBM), N-Propyl Mercapton (NPM). Isopropyl Mercapton (IPM), Methyl Acrylate (MA), Ethyl Acrylate (MA).
• In many CGS set-ups, the dosing unit of mercaptan is kept at 9 mg/m3. This unit is directly connected to the main line, after the metering skid.
• The odorization compound is highly corrosive & the quantity of odorant reduces downstream; however smell should be there for leakage detection.
• The odorant is stored in a storage tank and specific parameter are used to decide the size of the tank. The parameter which are important to decide the size of the tank are filling rate per year, gas flow, odorant injection rate, odorant density & whether to consider a single tank or dual tank.
• Generally the odorant tank is of 500 litre capacity. However, in some facilities, two 200 litre capacity tanks are used, one active and other standby.
• The odorant tanks are generally outdoor and the odorant passes through a bank & then to a barometer to measure the dosing pressure. Before passing the odorant downstream, the pipe thickness measurement is done.
• Pipe integrity check with colour coating is done as the mercaptan can get absorbed in the steel pipe wall, which can lead to pipe corrosion and fading of the mercaptan.
• The minimum odorant concentration which is calculated as Cn = (100K) / (0.2 LEL) mg/m3. Typical value of K of commonly used odorant is 0.03.

Types of Odorizers:

Wick-Type Odorizers - 

A Wick-style odorizer is one of the oldest types of odorization equipment. It is straightforward, inexpensive, and easy to use. It is perhaps the only way to odorize a single customer and is known as the “farm tap” odorizer. They are usually small, and the odorant is contained in a small bottle and installed on the service line near the gas meter. A cotton wick is inserted into the odorant and is saturated. The opposite end of the wick extends into the gas stream. The odorant draws up the wick and evaporates into the gas. Setting the dosage on this style of odorizer is performed by varying the wick size. The wick-type odorizer is only useful for odorizing individual lines such as a farm tap. The odorant usually recommended for wick odorizers is thiophane, which smells different than mercaptan. For this reason, at times when the town uses mercaptan in their distribution system, a diluted mercaptan may be utilized

 Bypass Odorizer - 

The bypass odorizer is also a simple and inexpensive method for a small system and is still used today. In this process, a portion of a gas stream diverts to a tank that contains liquid odorant. Most of these systems even include wicks to increase vaporization. The amount of gas diverted is dependent on the flow in the pipeline. The diverted gas flows on top of the odorant and leaves the tank, saturated, returning to the gas pipeline, thus odorizing it.

Gravity Feed Drip Odorizer - 

In gravity feed drip odorizer, the odorant dripping into the gas stream is controlled by a needle valve & monitored through a peep hole.There is clogging in needle valve at times due to variation of viscosity, density or odorant deposits.In modern days, control electronic smart grid systems are available in the market, which improve the process.

Injection type system - 

1. Injection system is a positive injection of the odorant which is stored away from the pipeline into the flowing stream. This system can be used on a wide range of flow rates & the system sizing is dependent on the injection size of the injection pump.
2. The system always operates in a proportion-to-flow manner. The odorant is injected in proportion to the flow rate of the natural gas flowing in the pipeline.
3. A ratio between the injected odorant and the flow of natural gas is measured in pounds of injected odorant per million cubic feet of natural gas flow.
4. A gas supply is taken from the flow stream, properly regulated and then communication to the injection system pump for use as an actuation.
5. The pumps used in this system can be diaphragm type or plunger type pneumatically driven pumps.
6. In a diaphragm type system, as the pump actuates, a hydraulic fluid is compressed against a diaphragm type seal containing a known volume of odorant.
7. This compression creates a pumping action on the odorant displacing the odorant volume into the flowing gas in the pipeline.
8. In the pneumatically driven pump system, the pump is operated by electronic system on the basis of gas flow rate. The pumps are suitable for flow rates above 5000 m3/hr.
9. The volume of each injection can be manually adjusted to change the amount of odorant injected with each stroke of the injection pump. Injection rate controller receives flow rate information from a flow measurement device.
10. Using a microprocessor. it automatically calculates the pump injection frequency required to meet a programmed injection rate.