Sun. Jun 20th, 2021

City gas distribution

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#71 Odorization Unit

9 min read
  1. Natural gas by nature is odourless which makes it impossible to detect when leakage occurs.
  2. In order to impart smell, odorant is added to the gas before it enters the medium pressure distribution system.
  3. 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.
  4. Generally odorant systems are installed at CGS, where custody transfer of gas takes place.
  5. 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.
  1. Ideally odorants have strong odour and must remain traceable as long as fault of technical equipment is detected.
  2. Odorant should be chemically stable and must have high vapour pressure in order to avoid condensation at operating pressure.
  3. The odorant should not react with gas componentss, piping material, rust, etc.
  4. Odorants must not contain water and must not be diluted with water due to possible subsequent corrosion of the equipment.
  5. After odorization, gas at a concentration as low as 1% by volume, should be detectable.
  6. 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.
  1. 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.
  2. 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.
  3. 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).

  1. 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.
  2. The odorization compound is highly corrosive & the quantity of odorant reduces downstream; however smell should be there for leakage detection.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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: Odorizers are introduced into gas stream by two different processes viz. Chemical Absorption & Chemical Injection. The principle of chemical absorption systems is the diffusion of odorant into a flowing natural gas stream by taking advantage of the chemical characteristics of the odorant vaporizing into the natural gas. In the chemical injection process, injection of an odorant is directly done into the flowing stream. Injection process is useful for wide ranges of flow rates. In the vaporization process, the oldest & simplest form of odorizing is by the help of wick odorizers.

a) Wick odorizers:

  1. There are two types of wick odorizers viz. adjustable & non-adjustable.
  2. In non-adjustable wick odorizer, odorant evaporated from the wick goes into the gas stream.
  3. The method is used for odorization of gas streams with lower flow rates.
  4. The device consists of a storage tank with an odorant, into which the wick is extended.
  5. The other end of the wick is placed directly into the gas stream. The dosage is controlled by changing the size of the wick.
  6. In these system, if gas flow rates are low, gas can be over odorized. On the other hand, if a gas flow rate is high, the intensity of odorization could be insufficient. This problem can be overcome by using an adjustable wick system.

Pulse bypass system:

  1. Pulse bypass system was first used by Peerless in 1938, which used gas meter and differential pressure.
  2. It has a vertical & a horizontal odorant tank which is not completely filled with odorant. It has an open space which is commonly referred as head space. The head space area is the junction where un-odorized gas enter into the tank.
  3. The gas rapidly absorbs odorant vapours coming from the liquid odorant that has high propensity to vaporize.
  4. The concentrated odorant vapour gets pushed out of the liquid tank into the pipeline. Gas saturated with odorant leaves the tank & goes back to original gas pipeline.

Gravity feed drip odorizer:

  1. In gravity feed drip odorizer, the odorant dripping into the gas stream is controlled by a needle valve & monitored through a peep hole.
  2. There is clogging in needle valve at times due to variation of viscosity, density or odorant deposits.
  3. 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.
  1. The pumps used in this system can be diaphragm type or plunger type pneumatically driven pumps.
  2. 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.
  3. This compression creates a pumping action on the odorant displacing the odorant volume into the flowing gas in the pipeline.
  4. 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.
  5. 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.
  6. Using a microprocessor. it automatically calculates the pump injection frequency required to meet a programmed injection rate.

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