Table of Content
- LPG Properties
- LPG Storage Rules
- LPG Handling Rules
- Sources of Ignition in LPG Plants
- Potential LPG Leakage Source in LPG Plants
Liquefied petroleum gas (LPG) is a fuel gas that contains a flammable mixture of hydrocarbon gases, specifically propane, propylene, butylene, isobutane, and n-butane. LPG is used as a fuel gas in heating appliances, cooking equipment, and vehicles(autogas). LPG is an environmental fuel that is relatively easy to reach and cheaper than other fossil fuels. LPG basically consists of butane and propane gases.
In fact, chemically these two gas types are very similar to each other, the main difference between them is the difference in their boiling points. Propane has a much lower boiling point than Butane. Butane and propane have different vapor pressures due to the difference in boiling points. Vapor pressures also play an important role in the storage conditions of LPG.
LPG has many advantages in this case. However, if there are no knowledgeable and experienced teams in storage and handling operations, LPG will create like a bomb. LPG is a very important fuel type in the global warming problem. From this perspective, LPG is part of the solution to global warming, not the problem. As is known, LPG terminals are operated with hazardous processes.
For this reason, reliability and safety are the most important parameters for LPG operations. There are 3 rules for safe operation at LPG terminals. The 1st rule is to have correct operational procedures, the 2nd rule is to create a strong maintenance culture, especially periodic inspection and testing of work equipment, and the last and most important is working with experienced and knowledgeable employees with a proactive safety culture.
What are the Properties of LPG?
LPG is a colorless liquid that readily evaporates into a colorless and odorless gas. Normally smelling ethyl mercaptan is added as an odouriser to LPG so that leaks can be easily detected. During leakage, the vaporization of liquid cools the atmosphere and condenses the water vapor contained in them to form a whitish fog which may make it possible to see an escape of LPG.
1. Density:
LPG at atmospheric pressure and temperature is a gas that is 1.5 to 2.0 times heavier than air. It is readily liquefied under pressure. The density of the liquid is approximately half that of water and ranges from 0.525 to 0.580 in 15 deg. C. Since LPG vapor is heavier than air, it would normally settle down at ground level/low-lying places, and accumulate in depressions.
2. Vapor Pressure:
The pressure inside an LPG storage vessel/ cylinder will be equal to the vapor pressure corresponding to the temperature of LPG in the storage vessel. The vapor pressure is dependent on temperature as well as on the ratio of the mixture of hydrocarbons.
3. Odour:
LPG has only a very faint smell, and consequently, it is necessary to add some odourant, so that any escaping gas can easily be detected. Ethyl Mercaptan is normally used as a stenching agent for this purpose.
4. Toxicity:
LPG even though slightly toxic, is not poisonous in the vapour phase, but can, however, suffocate when in large concentrations due to the fact that it displaces oxygen. In view of this, the vapour possesses mild anaesthetic properties.
LPG Storage Rules
When storing LPG in tanks, LPG should never be filled to 100 percent in the tank. For this purpose, the NFPA58 standard defines the maximum amount of volume for filling of LPG in the table. Looking at the table in NFPA58, the density and temperature of LPG are the 2 main parameters. There are 2 tables for tanks larger and smaller than 4.5 m3.
The filling amount of LPG in tanks by volume decreases as the density of LPG and LPG temperature decreases.
What is meant here is if the LPG temperature is low at the time of filling over time it will be subjected to heat transfer to reach the ambient temperature. As LPG will warm up in time, thermal expansion will occur of Liquid LPG. There should be sufficient vapour space in the tanks so that thermal expansion can occur properly.
LPG Storage Tank Farms should provide some important points.
- Fire fighting, water sprinkler system and deluge system: Fire fighting systems should be designed according to NFPA 58, they can fight in possible fire scenarios of the facility and should also be activated automatically. 3 parameters are critical in fire fighting systems: the amount of storage water, water flow rate of fire pumps and number of pumps.
- Gas leak detection system: In gas detection systems, the design of gas detectors should be planned considering the prevailing wind directions of the plants and potential sources of leaking.
- Comprehensive emergency systems: In emergency situations, action should be taken to protect the entire facility. First of all, all LPG operations should be stopped rapidly and the emergency shut-off valves of the tanks should be closed (LPG source must be cut). If there is a fire situation, simultaneous fire fighting equipments must be activated for emergency team can fight fire Finally, entrance/exit gates should be opened automatically so that people can leave safely and vehicles such as fire brigades and ambulances can enter. Of course, during this process, sirens should be sounded in a way that does not prevent communication, and everyone around should be informed of the emergency situation.
- Tank gauging system to prevent accidental overfill: High-level alarm system to sound alarm when the level in the tank reaches max liquid level. API recommendation regarding overfilling is to install level alarms independent of level gauge systems. Also, Gauging systems should provide high temperature and over-pressure alarms.
- Safety valves located in LPG Plants: Pressure safety valves on the tank should be opened to safe zones and should never impact the shell of the neighboring tank. Pressure safety valves or thermal relief valves in the pipeline should not placed under the tank, When the safety valve is open it should not impact the tank shell.
- Emergency stop buttons at strategic location: Emergency stop and fire alarm buttons should be located in areas accessible to plant personnel. Additionally, when the button's cables are broken or burned, the emergency system should be activated as if the button was pressed.
LPG Handling Rules
LPG occurs in two phases, the first is vapor and the second is liquid. Compressors are used when handling LPG in the vapor phase, pumps are used when handling it in the liquid phase.
Vapor LPG Handling:
The LPG vapour phase is handled by compressors, there are some important rules when handling the vapor phase, the first of which is to prevent liquid from coming to the compressors, in this respect, liquid traps should be installed at the inlets of LPG compressors If liquid trap detected liquid LPG, the compressor should be shut down.
In addition, compressors compress the vapor phase, ıt suction from the inlet and pressure to the outlet of the vapor line, this compression process will increase the temperature as well as the pressure of the vapor phase. These temperatures should be monitored, temperature is a very important parameter. In this respect, for example, if rubber LPG hoses are used in the vapor phase discharge line, the maximum operating temperature should not be exceeded. Generally, the rubber LPG hose will be dangerous when the maximum exceeds 70 degrees C.
Liquid LPG Handling:
When handling liquid LPG, pumps are used, pumps are complex and powerful equipment. If the LPG pump runs without carefully, it may cause the LPG to fire. Because pumps may be ignition sources.
The most critical equipment of pumps is their mechanical seals. To ensure continuity of sealing, mechanical seals must be selected correctly and maintained on time.
The second critical issue for pumps is preventing them from running dry. If the pumps run dry, the sealing elements will be damaged first. Damaged seals create leaking of LPG. Should protect seals on pumps for process safety. To prevent dry running, liquid sensors should be installed in the pump suction lines. If there is not enough liquid in the pump suction line, the pump must stop immediately.
Sources of Ignition in LPG Plants
Smoking:
There are frequently local regulations about smoking which must be rigidly observed. Smoking may be permitted but only under controlled conditions at times and in places specified by the procedures. Personnel when working plants must not carry matches or more particularly lighters, and the risk of doing so is to be impressed on all.
Portable Electrical Equipment:
Only ATEX-approved Safety torches or hand lamps are to be used. Portable Electric Equipment self-contained or on extension cables are not to be used in a gas-dangerous place or zone unless the equipment is intrinsically safe. Portable domestic radios, electronic calculators, tape recorders and other non-approved battery equipment are not to be used in a gas-dangerous place or zone.
Hot work:
Before any hot work, hammering, chipping, or power tools including shot blasting are used the responsible HSE worker is to examine the area to be worked and satisfy himself that such work can be safely undertaken and a hot work permit certificate issued. Non-sparking tools are not to be used as they do not significantly reduce the risk of igniting a flammable vapour.
Auto Ignition:
The vapours from LPG may ignite if the vapour comes into contact with any surface heated above the auto-ignition temperature e.g. steam lines, exhaust manifolds, or overheated equipment.
Static Electricity:
Static electricity can cause fires or explosions—the energy that a static spark gives off is enough the amount necessary to ignite LPG.
Potential LPG Leakage Source in LPG Plants
Flanges of LPG Pipeline and Vessel:
Flanges are equipment that may leak due to their nature. In this regard, the gaskets and bolts used in the flanges must be selected correctly. The most basic safety in flanged connections is to tighten the bolts with sufficient torque and use fire-proof gaskets. Leakage checks should be done with soapy water on the flanges periodically, especially during periods when the atmospheric temperature rapidly changes (seasonal transitions).
Rotating Equipment Mechanical Seals:
Mechanical seals are used as sealing equipment in pumps, in this respect, it is recommended to use double mechanical seals in LPG pumps. Double mechanical seals are designed to ensure maximum sealing safety. These seals virtually eliminate leakage of the fluid being handled in pumps.
Pressure Safety Valves:
Pressure safety valves are actually designed as safety equipment, protecting tanks and pipes against overpressure. However, sometimes if pressure safety valves are faulty, they may activate under the set values and release LPG into the atmosphere. this equipment is very critical in LPG operations, so for this reason, LPG safety valves should be tested and maintained at least every 5 years.
LPG Sampling Point:
Sampling is an operational requirement for quality control in LPG operations. LPG sampling should be done according to procedures, otherwise it may cause large LPG releases. The adequacy of safety barriers in the LPG sampling zone should be checked, The important safety barrier is that a two-valve system should be installed, and the last valve should be a spring-loaded dead man valve.
Truck Loading and Unloading Connection Couplings:
During LPG loading and unloading, couplings are connected and removed to trucks and ship manifolds. While doing this operation, some LPG is released, this is a controlled and known amount of released LPG. The critical point in these operations is the uncontrolled release that may occur during connecting and removal. In order to prevent this situation from occurring, it is necessary to check the couplings periodically. In this regard, the first important rule is the control of the connection couplings with go-no-go gauges, the second important rule is to use breakaway or pullaway couplings, both pieces of equipment must be maintained and checked on time.
Water Dreyn of LPG Vessel:
The water removal and drainage process is potentially the most hazardous process in LPG operations. There have been many accidents in the past regarding this issue. API has made some recommendations on this issue; the two-valve system should be in the drain lines. However, accident examples have shown that in addition to just two valves, there must also be a spring-loaded dead man valve. Ayrıca There should be one valve that can remotely close the drainage systems under the tanks in case of emergency.
