Flare valves

As flow rate to the compressor increases, the suction pressure rises until the volume of gas at actual conditions of temperature and pressure compressed by the cylinder equals the volume required by the cylinder. A flare valve is needed to keep the suction pressure from rising too high and overpressuring the suction cylinder, creating too high a rod load or increasing the horsepower requirements beyond the capability of the driver (see Chapter 11 for further discussion). 

The flare valve also allows production to continue momentarily if a compressor shuts down automatically. Even in booster service it may be beneficial to allow an operator to assess the cause of the compressor shutdown before shutting in the wells. In flash gas or gas-lift service, it is almost always beneficial to continue to produce the liquids while the... 

The pressure upstream of the suction valve will increase until sufficient back-pressure is established on the wells or equipment feeding the compressor to reduce the flow to a new rate in equilibrium with that being handled by the cylinder or until a flare valve or relief valve is actuated. 

Suction throttle valves are common in gas-lift service to minimi/c the action of the flare valve. Flow from gas-lift wells decreases with increased back-pressure. If there were no suction valve, the flare valve may have to be set at a low pressure to protect the compressor. With a suction valve it may be possible to set the flare valve at a much higher pressure slightly below the working pressure of the low-pressure separator. The difference between the suction valve set pressure and the flare valve set pressure provides a surge volume for gas and helps even the flow to the compressor. 

A speed controller does not ehminate the need for a recycle valve, flare valve, or suction throttling valve, but it will minimize their use. The recycle valve and suction throttling valve add arbitrary loads to the compressor and thus increase fuel usage. The flare valve leads to a direct waste of reservoir fluids and tlius loss of income. For this reason, engine speed control is rec-... 

As suction pressure increases or discharge pressure decreases, the compressor head requirement will decrease and the flow rate will increase. A flare valve will avoid stonewalling or overranging driver horsepower. 

During normal operation the flare valve is closed and the pressure in the gasifier is controlled through a by-pass valve at the booster compressor. Quite naturally is the gasifier a slow system concerning both pressure and temperature control. The output control of the gas turbine is completely different and it responds more or less instantaneously. Operation in the fully integrated mode made the pressure, temperature and gas quality in the system vary a bit when the gas turbine suddenly compensated for a small change in either parameter. 

The pilot oil burner on the flare can be operated manually or automatically with it starting and stopping as the flare valve opens and closes. 

Minimization of differences ( deltas ) between end-of-line flare meter readings and baseline purge rates (typically attributable to small flaring sources) through flare valve surveys. 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

Vents and flares are intended to take contaminants released from safety valves away from work areas. However, if an elevated vent is at the level of an occupiable platform on the same or an adjacent unit, a worker may, under certain wind conditions, be subject to the nearly undiluted effluent of a vent. Whereas such elevated platforms may rarely be occupied, a heavy exposure from a vent could incapacitate a worker or cause a fall. Tanks that vent only when being filled are common causes of this concern. The usual solution is to raise the vent above any occupiable platform or, at greater cost, to scmb the vent effluent. 

Parts made from fluoroelastomers ate used ia appHcations that justify their high cost, usually where the maintenance and replacement costs are high enough to offset the initial cost of the part. These include automotive appHcations such as valve stem seals, fuel injector components, radiator, crankcase and transmission seals, and carburetor needle tips. Numerous seals and gaskets in the marine, oilfield, and chemical processing industries employ fluoroelastomers. In addition, many hoses in the automotive and chemical industry are made entirely of fluoroelastomer compounds or have a veneer of the fluoroelastomer as a barrier exposed to the harsh environment. Seals and gaskets in military appHcations and the binder for flares and missile appHcations ate made with fluoroelastomers. 

Another pressure control method is a hot gas bypass. The author has seen a retrofitted hot gas bypass system solve an unsatisfactory pressure control scheme. In the faulty system, a pressure control valve was placed between the overhead condenser and the lower reflux drum. This produced en atic reflux drum pressures. A pressure safety valve (psv) vented the reflux drum to the flare to protect against over-pressure. A psv saver control valve, set slightly below the psv relieving... 

Mak has developed an improved method of relief valve manifold design. The APT has adopted this method, which starts at the flare tip (atmospheric pressure) and calculates backwards to the relief valves, thus avoiding the trial and error of other methods. This is especially helpful when a large number of relief valves may discharge simultaneously to the same manifold. 

Starting at the flare tip calculate logical segments using Figure 1 until all relief valve outlet pressures are found. 

Often, if both high and low pressure relief valves need to relieve simultaneously, parallel high and low pressure headers terminating at the flare knockout drum are the... 

Safety relief systems are verified as part of PSM. This includes the PS Vs themselves and also flare system piping networks. Safety relief valves are covered in Section I—Fluid Flow. A good procedure for sizing the flare system piping is found in Section 19—Safety-Relief Manifolds. This method, first published in the Oil and Gas Journal, has been adopted by APl. I have also used... 

For the tandem arrangement gas seal, a primary seal vent must be pro vided to vent the leakage across the process side seal. This vent ma> lie to flare or other suitable gas disposal point. The back pressure under nor mal conditions should be kept to a low value. A small amount of back pressure is recommended to keep a positive differential across the see ondaiy seal. Leakage measurement may be provided in the vent line to provide health monitoring of the primary seal. Unfortunately, the rotameter, which would be the obvious choice, should not be used because of its lack o reliability. If an orifice or needle valve is used to set the back pressure to the seal vent, pressure upstream of the restriction can be measured for a relative flow measurement. This type of reading does provide trend data that may be used to judge the seal s performance. 

This chapter describes the basic principles and procedures for the evaluation of overpressure potential in plant equipment, and for the selection, design and specification of appropriate pressure relieving facilities. The design of closed safety valves and flare headers is included in this chapter, but blowdown drums and flares are covered separately. To properly discuss this subject, the reader should become familiar with the following terminology. 

Closed Disposal System - This is the discharge piping for a PR valve which releases to a collection system, such as a blowdown drum and flare header. However, a closed system can also be a process vessel or other equipment at a lower pressure. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

In offsite locations, thermal expansion PR valves may discharge to a flare header upstream of a knockout drum, if available, or to the equipment (e.g., a tank) on the opposite side of one of the blocking-in valves, or to the atmosphere. Atmospheric discharges must be at grade level in a safe location... 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

A check valve is not, however, permissible in PR valve inlet or outlet piping, or in any flare or safety valve header. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

It should be noted that the above maximum radiant heat density criteria for application to inadvertently ignited atmospheric releases from pressure relief valves or vents are less restrictive than those used for flare design. This results from the fact that flares are continuously ignited, whereas ignition of a relieving PR valve is unlikely. In addition, the area surrounding a flare is open and offers no protection, while within a process unit access to shelter is available. 

Pumps and Furnaces - It is not always necessary for a PR valve which must discharge to a closed system to be tied into a flare header. For example, PR valves on furnaces frequently discharge to the vessel downstream of the furnace, and PR valves on pumps normally discharge to the pump suction or pump suction vessel. 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

In adxlition to handling PR valve releases, the flare header is also used to route certain other emergency releases to the blowdown drum. These include drainage from fuel gas, compressor and absorber knockout drums. 

The factors affecting the sizing of a closed release system to the flare are described below. The same principles apply to the sizing of other closed release systems e.g., PR valve releases which are manifolded into an atmospheric vent. 

Routing of Flare Header through Process Areas - Flare headers in process areas should be routed to avoid locations of particularly high fire risk, such as over pumps, near furnaces, etc. The headers and subheaders should also be laid out and provided with isolating CSO valves and spectacle blinds, unless prohibited by local codes, such that it is not necessary for flare lines to remain in service in units which are shut down separately. Blowdown and water disengaging drums should be spaced from process areas. 

When applying low temperature requirements, one should consider safety valve and flare headers to be subject to "shock chilling" if they can be exposed to cold liquids released into the system. This includes flare headers from blowdown drums into which cold liquids are discharged. Where laterals of different piping material are combined, the material of the lower-temperature header is continued for the rest of the combined line, and is also extended back into the other lines for 6 m. 

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