Solutions for a volatile problem

A lack of infrastructure for natural gas in some parts of the world has brought on a search for alternative clean, efficient fuels. Naphtha is a promising candidate, but until recently, its volatility and flammability presented a design challenge for fuel treatment systems.

Mats Englund,

Alfa Laval Marine & Power,

Tumba, Sweden

In the search for economical, efficient and environmentally acceptable liquid fuels for gas turbines, the class of volatile fuels has attracted an increased interest from the power generation industry. The pool of volatile fuels comprises liquid hydrocarbons such as naphtha, gas condensate and kerosene.

The fuels market for gas turbines and diesel engine driven power plants is changing. While natural gas remains the primary fuel for gas turbines, the supply of liquid fuels has diversified. In addition to middle distillates and heavy fuel oils (HFO), the use of crude oils, natural gas liquid (NGL), naphthas and kerosene is increasing. The main regions concerned are Southeast Asia and India. In the long term, natural gas is the preferred fuel in these areas, but until gas supply and distribution can be arranged nationwide, liquid fuels will continue to play a role.

The biggest potential for the use of naphtha and gas condensate in gas turbine power plants exists in India. With a void of a developed infrastructure for supplies of natural gas and increased competition in middle distillates from the transportation sector, the Indian power industry is looking for alternative products among which naphtha is a promising candidate.

The Indian government has in fact approved 12 000 MW generated by liquid fuels up to 2002. About 70 per cent of this could be covered by naphtha. At the time being there are three power stations operating on naphtha in India. The Dabhol plant is today generating 600 MW and will contribute with a total of 2600 MW in full operation 1999.

The price relations and development of price over time are illustrated in Figure 4. A surplus of world supply of naphtha is forseen up to year 2002, keeping world prices under pressure.

The chemical properties of volatile fuels favour efficient combustion in heavy duty gas turbines. However, due to their high volatility they have highly flammable properties that require extra caution during handling, treatment and storage. These physical characteristics have therefore presented a challenge for the design of reliable equipment such as centrifugal separation systems which can produce contaminant-free fuel without risk.

Volatile fuel

From a performance point of view, naphtha can readily be treated in centrifuges with excellent separation results. Water contamination of naphtha, including salts and ash-forming components, typically occurs during transport and storage, and are removed by centrifugal separation. However, the volatile properties causes concern about safety and operational aspects.

The physical and chemical characteristics of naphtha are therefore important for the design of fuel treatment systems in terms of separation performance and safety.

Separation: The typical density range for naphtha is 0.69 to 0.79 g/ml at 15 degrees C, this being remarkably lower than middle distillates and light fuels. The low density helps separation, the centrifuge readily removes water contamination that has not already settled in the storage tank.

The viscosity of naphtha is less than 1 cSt measured at 20 degrees C and has little influence on separation performance in the centrifuge. However, viscosity is linked to lubricity which is a critical property of naphtha, the low lubricity causing abrasive wear on reciprocating components with tight clearances. For this reason it is commonly recommended to use lubricity improving additives with naphtha.

Sediments in the refined product are present in levels of less than 5 ppm, however higher levels of sediments are likely to appear due to imperfections during handling. The centrifuge removes sediment to a high degree. The content of water soluble trace metals sodium and potassium and oil soluble vanadium in the pure naphtha from the refinery is less than 0.5 ppm.

Safety: The flash point indicates the temperature where the vapours from a liquid are ignited by a nearby located flame source. The flash point is used to assess explosion risks and to classify combustible products. In the design of centrifuge systems the flammability class and the operating temperature are used to determine the required explosion protection of the system. Separation of flammable liquids according to Class IA at temperatures above the flash point requires inert gas blanketed systems.

Knowledge about the Reid vapour pressure will give a picture of the volatility of the fuel. Data on vapour pressure is used to estimate risk for cavitation and explosion.

Lower and upper flammability limits define the concentration range (vol.% gas in air) where a flame will propagate through the whole gas mixture. Naphtha and gas condensate have a wider flammability range than less volatile fuels, so the explosive limits are reached at lower temperatures and concentrations. This implicates that inert gas protected equipment must not allow for any gas leaks to the surrounding atmosphere where an explosive mixture could form with the air.

Design considerations

The specific properties of naphtha require special concern when designing a cleaning system. This takes into consideration that two essential conditions must co-exist before an explosion can be initiated: the vapours must be within the flammable limits; and a source of ignition must be present.

Hazardous area zones are defined according to the probability of a flammable atmosphere being present. The selection of equipment for hazardous areas must fulfil the requirement of the zone. For electrical equipment there are standards to specify type and design of explosion protection. Mechanical equipment is divided into rotating and non-rotating equipment. The non-rotating equipment has no classification; rotating equipment like centrifuges is classified according to zones.

•Zone 0: a zone in which a flammable atmosphere is continuously present. Rotating equipment is not permitted.

•Zone 1: a zone in which a flammable atmosphere is likely to occur in normal operation. Rotating equipment must be gas tight, non-sparking, temperature according to temperature class.

•Zone 2: a zone in which a flammable atmosphere is not likely to occur during normal operation. Rotating equipment must be non sparking, surface temperature according to temperature class.

•Non-hazardous area: an area not classified as zone 0, 1 or 2. Surface temperature according to temperature class.

System design

Alfa Laval has designed a new centrifugal separation system for naphtha – GTN30 – which produces a refined fuel virtually free of contaminants and which meets all applicable safety codes. To comply with these standards, the system design includes a number of modifications compared to standard fuel centrifuges:

•Replace friction clutch with direct drive

•Replace mechanical brake with pneumatic or electronic dc brake

•Use a suitable explosion proof centrifuge drive motor with capability for controlled torque starting

•Star/Delta motor starter to manage the high torque start-up acceleration

•Local control unit located in suitable EEx proof cabinet. Otherwise placed into non-hazardous area

•All instruments and control items located in hazardous zone in EEx Proof execution

•All wiring, circuitry, connectors and glands of suitable EEx Proof type

•Use of inert gas blanketed or purged centrifuge system warranted by fluid conditions.

Alfa Laval`s naphtha centrifuge system GTN30 is divided into functional blocks each designed to fulfil the specific demands for explosion protected design.

The separator GTN30 is a high speed centrifuge with a solids discharge mechanism, discharging a part of the bowl solids content at certain intervals from the periphery of the bowl through the solids outlet. The centrifuge is set up for purifier operation with separate outlets for separated water and cleaned naphtha fuel.

The separator has connections for process media, flushing liquid, cooling liquid and inert gas. There are also ventilation and drain outlets.

The process fluid module is used for flow control and supply of safety liquid. There is one inlet/outlet for process and cleaning liquid, another for inlet/outlet for safety liquid. Inlet and outlets are equipped with pneumatic valves in order to accomplish automatic changeover between different operating conditions. A liquid seal in the outlet line is used to maintain a predetermined inert gas over-pressure in the system.

The service liquid module supplies service liquid to the separator. It has connections for flushing liquid and operating liquid. Flushing is used to prevent solids build up in the frame and outlet cyclone. Operating liquid is used to manoeuvre the opening and closing of the bowl.

When the separator is running on flammable liquid such as naphtha, a dangerous concentration of flammable vapours in the bowl casing can be reached, leading to an explosion.

It is difficult to control the vapour concentrations and eliminate the risk of an ignition source. Most practicable is to control the oxygen concentration. An oxygen concentration below the minimum, which will allow ignition of any concentration of the vapours, is safe. Operation at an oxygen concentration below five per cent at all times is considered safe. A proven method to limit the air and oxygen concentration in a confined space is to replace the air with an inert gas. Nitrogen is commonly used, as it is reasonable in cost and readily available.

The control equipment supervises the entire system and gives alarms and actions if anything goes wrong. The supervision includes the separator motor and dc brake, inert gas supply, vibration level, bowl speed, product flow, the discharge performance and safety liquid supply.

Before start the system is purged with inert gas to reduce oxygen to a level of two per cent. The required volume of inert gas is normally about five times the volume of the system. The start of separator is blocked until the initial purging has been completed. During purging the control system checks that the inert gas flow is above a certain value for a certain time. If this condition is true the purging is automatically stopped and a signal is given that the system is inerted.

After purging, a slight over-pressure is maintained by a low flow of inert gas. All gas outlets from the separator are connected to a liquid seal giving a stable over pressure to the system. The flow of inert gas is small, mainly needed to compensate for inert gas losses in the system.

Should the inert gas pressure fall below a certain limit, more gas is supplied through the purge gas supply line. If the gas pressure decreases further below a minimum limit, the inerted condition is considered lost and the separator is stopped.

It is important for the function of the system to limit the losses of inert gas. One example for such loss is if the paring disc pumps out gas with the naphtha fuel. To avoid this, a back-pressure is applied to the separator outlet line.

Explosion testing

A number of theoretical risk analyses have been conducted on separation of flammable liquids in centrifuges, however very few practical tests are reported. For this reason Alfa Laval initiated and executed a series of tests in co-operation with the Norwegian research institute Christian Michelsen Research in Bergen, Norway.

The investigation aimed to ascertain under which conditions flammable atmospheres are present in centrifuges and what the consequences of a gas explosion in the centrifuge would be. The test results were used to set demands for our explosion proof separation systems considering how to completely avoid explosion risks and how an accidental gas ignition can be mitigated (quenched) before it propagates through the full gas volume and causes an explosion.

It was found that during certain conditions, a flammable atmosphere is present inside the separator frame, when the separator runs on a volatile liquid. Observations during worst case explosion tests showed that the centrifuge was capable of containing the forces from the explosion without throwing parts around or attaining a dangerous running mode.

Conclusions from the tests were that it is possible to avoid a flammable mixture inside the separator frame either by use of inert gas or by a modified ventilation and centrifuge design. It was also concluded that mitigation of gas explosions is possible by quenching with the use of flame arresters and suitable design of the interior of the separator frame. It seemed that the serious consequences of an explosion would not be the mechanical damages to the separator but the risk of extensive fire when flammable liquid is spread out.

The tests provided increased knowledge about explosion hazards in centrifuge systems and confirmed that these technical solutions are safe for the treatment of flammable liquids.

Optizoom improves separation efficiency

Alfa Laval Marine & Power will this month launch Optizoom, a new fuel treatment system for liquid fuels used in gas turbine and combined cycle power plants.

The Optizoom system will be used in the company`s latest GT range of separators and is designed to ensure that fuel reaches the gas turbine at the level of cleanliness, flow, temperature and pressure specified by the gas turbine manufacturer.

The new system is composed of two main elements, a GT separator and a chemical additive lubricity conditioner – called Alpacon – which enhances separation performance. All of Alfa Laval`s separators in the GT range will be equipped with Optizoom technology.

The forces acting on liquid fuel in a centrifugal separator can be considerable, especially when increasing in speed from zero to the rotational speed of the separator bowl. This can lead to problems such as the formation of small droplets, air mixing and foaming.

Optizoom technology counteracts these problems by allowing smooth acceleration of the liquid before it enters the conical disc stack. In addition, the use of Alpacon improves water coalescence. The overall result is a high separation efficiency.

The Alfa Laval GT range includes the GT25, GT40 and GT60 designed for gas turbine heavy fuel oil and distillate, and the GTN30 designed for volatile fuels such as naphtha and gas condensates. This range also carries new designs for the bowl, paring disc, operating water module, and inlet and outlet connections.

Click here to enlarge image

Figure 1. The GTN30 centrifugal separator

Click here to enlarge image

Figure 2. The GTN30 with inert gas ancillaries

Click here to enlarge image

Figure 3. Flammability ranges

Click here to enlarge image

Figure 4. Market prices of naphtha. A surplus of world supply is forseen until 2002. (Singapore spot prices )

Click here to enlarge image

Figure 5. Naphtha treatment system and zone requirements