ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 10 pptx

To drain the vessel, the throttling valve is shut and one or more drain valves are opened.. To stop draining, thethrottling valve is closed, flow goes to zero, and the drain valves are s

Valves, Fittings, and Piping Details 461

(text continued from page 453)

Insulation for personnel safety is required only when accidental tact of the hot surfaces could be made by personnel within normal work

con-or walk areas Isolation may be in the fcon-orm of guards con-or barriers and, inspecial cases, warning signs

Hot surfaces associated with natural gas compressors and pumps dling volatile flammable fluids should be insulated since the equipmentitself is a source of hydrocarbon liquids or gases Generators, electricmotors, and engine-driven equipment such as fire water pumps, wirelineunits, welding machines, hydraulic equipment, and the like do not them-selves cause the area to become classified from an electrical standpoint,However, they may be in a classified area due to other equipment andthus require insulation or barriers Turbo-chargers, exhaust manifolds,compressor heads, expansion bottles and the like (including associatedpiping), which cannot be insulated without causing mechanical failure,are not normally insulated In these cases, warning signs, barriers, gasdetectors, or other methods for the protection of personnel and minimiz-ing exposure to hydrocarbon liquids and gases are acceptable

han-MISCELLANEOUS PIPING DESIGN DETAILS

Target fees

Where 90° turns in piping are required, standard long radius ells (ellcenterline radius equals 1.5 times pipe nominal diameter) are usuallyused In sandy service, the sand has a tendency to erode the metal on theoutside of the bend Target tees, such as shown in Figure 15-25, are oftenspecified for such service The sand builds up against the bull plug andprovides a cushion of sand that is constantly being eroded and subject todeposition by the sand in the flow stream

Chokes

The flow of fluid leaving a choke is in the form of a high-velocity jet.For this reason it is desirable to have a straight ran of pipe of at least tenpipe diameters downstream of any choke prior to a change in direction,

so that the jet does not impinge on the side of the pipe

Often on high-pressure wells two chokes are installed in the line—one a positive choke and the other an adjustable choke Theadjustable choke is used to control the flow rate If it were to cut out, thepositive choke then acts to restrict the flow out of the well and keep thewell from damaging itself Where there are two chokes, it is good pipingpractice to separate the chokes by 10 pipe diameters to keep the jet offlow formed by the first choke from cutting out the second choke Inpractice this separation is not often done because of the expense of sepa-rating two chokes by a spool of pipe rated for well shut-in pressure It ismuch less expensive to bolt the flanges of the two chokes together Nodata has been collected to prove whether the separation of chokes is justi-fied from maintenance and safety considerations.

flow-Whenever a choke is installed, it is good piping practice to installblock valves within a reasonable distance upstream and downstream sothat the choke bean or disc can be changed without having to bleed down

a long length of pipeline A vent valve for bleeding pressure off the ment of the line containing the choke is also needed This is particularlytrue in instances where a positive choke is installed at the wellhead and

seg-an adjustable choke is installed hundreds of feet away in a line heater Ifblock valves are not installed downstream of the positive choke andupstream of the adjustable choke, it would be necessary to bleed theentire flowline to atmosphere to perform maintenance on either choke,

Valves, Fittings, and Piping Details 463

good spot for corrosion to develop, as shown in Figure 15-26 Flangeprotectors made of closed-cell soft rubber are sometimes used to excludeliquids from penetrating this area Stainless-steel bands and grease fit-tings are also used

Closed-cell flange protectors are much less expensive than stainlessbands However, if not installed properly they can actually accelerate cor-rosion if a path is created through the material to allow moisture to enter,Flange protectors should not be used in H2S service They may trap smallleaks of sour gas and keep them from being dispersed in the atmosphere

Figure 15-26 Flange protector types.

Vessel Drains

If vessel drain valves are used often, there is a tendency for thesevalves to cut out As the valve is opened and shut, there is an instanta-neous flow of a solid slurry across the valve that creates an erosiveaction Figure 15-27 shows a tandem valve arrangement to minimize thispotential problem To drain the vessel, the throttling valve is shut and one

or more drain valves are opened These valves open with no flow goingthrough them Then the throttling valve is opened To stop draining, thethrottling valve is closed, flow goes to zero, and the drain valves are shut.The throttling valve will eventually cut out, but it can be easily repairedwithout having to drain the vessel

Vessel drain systems can be very dangerous and deserve careful tion There is a tendency to connect high-pressure vessels with low-pres-sure vessels through the drain system If a drain is inadvertently leftopen, pressure can communicate through the drain system from the high-pressure vessel to the low-pressure vessel If this is the case, the lowpressure vessel relief valve must be sized for this potential gas blowbycondition

atten-The liquid drained from a vessel may flash a considerable quantity ofnatural gas when it flows into an atmospheric drain header The gas willfind a way out of the piping system and will seek the closest exit toatmosphere that it can find Thus, a sump collecting vessel drains must

be vented to a safe location

Figure 15*27 Drain valves for a separator.

Valves, Fittings, and Piping Details 465

Open Drains

Open, gravity drains should not be combined with pressure vesseldrain systems The gas flashing from vessel liquids may exit an opendrain system at any point and create a hazard

On open drain piping leaving buildings, a liquid seal should beinstalled as further protection to assure that gases flashing from liquidsfrom other locations in the drain system will not exit the system in thebuilding

The elevation of gravity drain systems must be carefully checked toassure that liquids will flow to the collection point without exiting thepiping at an intermediate low point

Piping Vent and Drain Valves

At high points in piping, vent valves are required to remove air forhydrotesting and for purging the system At low points, drain valves arerequired to drain liquids out of the system to perform maintenance Nor-mally, vent and drain valves are H-in or %-in ball valves

Control Stations

Whenever it is necessary to control the process level, pressure, ature, etc., a control station is installed A control station may be as sim-ple as a single control valve or it may contain several control valves,block valves, bypass valves, check valves, and drain or vent valves.Where there is a control valve, block valves are often provided so thecontrol valve can be maintained without having to drain or bleed thepressure from the vessel Typically, the safety-systems analysis wouldalso call for a check valve at this point to prevent backflow Drain or ventvalves are often installed to drain liquid or bleed pressure out of the sys-tem so that the control valve can be maintained In smaller installationsdrain and vent valves may not be provided and the line is depressured bybacking off slightly on flange bolts (always leaving the bolts engageduntil all pressure is released) or slowly unscrewing a coupling This is not

temper-a good prtemper-actice temper-although it is often used for smtemper-all-ditemper-ameter, low-pressureinstallations

Bypass valves are sometimes installed to allow the control valve to berepaired without shutting in production On large, important streams thebypass could be another control valve station Manual bypass valves are

more common The bypass valve could be a globe valve if it is

anticipat-ed that flow will be throttlanticipat-ed through the valve manually during thebypass operation, or it could be an on/off valve if the flow is to becycled Because globe valves do not provide positive shutoff, oftenglobe-bypass valves have a ball or other on/off vaive piped in series withthe globe valve

The piping around any facility, other than the straight pipe connectingthe equipment, is made up primarily of a series of control stations Flowfrom one vessel goes through a control station and into a piece of pipethat goes to another vessel In addition to considering the use of blockvalves, check valves, etc., all control stations should be designed so thatthe control valve can be removed and any bypass valve is located above

or on a level with the main control valve If the bypass is below the trol vaive, it provides a dead space for water accumulation and corrosion

Prime movers are typically fueled by natural gas or diesel Dual fuelturbine units exist that can run on natural gas and can automaticallyswitch to diesel So-called "dual fuel" reciprocating engines run on amixture of diesel and natural gas When natural gas is not available, theycan automatically switch to 100% diesel Most prime movers associatedwith producing facilities are typically natural gas fueled due to the readyavailability of fuel Diesel fueled machines are typically used to providestand-by power or power for intermittent or emergency users such ascranes, stand-by generators, firewater pumps, etc.

Due to the extremely wide variety of engines and turbines available,this discussion is limited to those normally used in production facilities.The purpose of this chapter is to provide facility engineers with an under-standing of basic engine operating principles and practices as necessaryfor selection and application The reader is referred to any of the manytexts available on engine and turbine design for more in-depth discussion

of design details

^Reviewed for the 1999 edition by Santiago Pacheco of Paragon Engineering Services, Inc.

467

RECIPROCATING ENGINES

Reciprocating engines are available in two basic types—two-stroke orfour-stroke cycle Regardless of the engine type, the following four func-tions must be performed in the power cylinder of a reciprocating engine:

1 Intake—Air and fuel are admitted to the cylinder,

2 Compression—The fuel and air mixture is compressed and ignited

3 Power—Combustion of the fuel results in the release of energy Thisenergy release results in increase in temperature and pressure in thecylinder The expansion of this mixture against the piston converts aportion of the energy released to mechanical energy

4 Exhaust—The combustion products are voided from the cylinderand the cycle is complete

In this manner the chemical energy of the fuel is released Some of theenergy is lost in heating the cylinder and exhaust gases The remainder isconverted to mechanical energy as the expanding gases move the piston

on the power stroke Some of the mechanical energy is used to overcomeinternal friction or to sustain the process by providing air for combustion,circulating cooling water to remove heat from the cylinder, and circulat-ing lube oil to minimize friction The remainder of the energy is available

to provide external work The amount of external work that can be oped by the engine is termed its "brake horsepower" or bhp The amount

devel-of work required to sustain the engine is termed its "friction horsepower"

or fhp The work developed by the power cylinders is termed the cated horsepower" or ihp The indicated horsepower is the sum of boththe friction horsepower and the brake horsepower

"indi-ihp = bhp + fhp

Four-Stroke Cycle Engine

The four-stroke cycle engine requires four engine strokes or 720degrees of crankshaft rotation to complete the basic functions of intake,compression., power, and exhaust All flow into and away from the cylin-ders is controlled by valves directly operated by a camshaft that is driven

at 1 A engine speed Figures 16-1 and 16-2 illustrate a cross section and an

idealized P-V diagram for a four-cycle spark-ignited engine, respectively

Prime Movers 469

Figure 16-1 Cross section of 4-cycle, spark-ignited engine.

Intake Stroke (Point 1 to Point 2)

With the intake valve open, the piston movement to the right creates alow pressure region in the cylinder, which causes air and fuel to flowthrough the intake valve to fill the cylinder

Compression Stroke (Point 2 to TDC)

The intake valve is now closed as the piston moves from the bottomdead center (BDC) to top dead center (TDC), compressing the fuel/airmixture At Point 3, just prior to TDC, a spark ignites the fuel/air mixtureand the resulting combustion causes the pressure and temperature tobegin a very rapid rise within the cylinder

Power Stroke (TDC to Point 4)

Burning continues as the piston reverses at TDC and pressure risesthrough the first portion of the "power" or "expansion" stroke It is theincrease in pressure due to burning the fuel that forces the piston to theright to produce useful mechanical power The piston moves to the rightuntil BDC is reached

% of Piston Displacement

Figure 16-2 Idealized P-V diagram for a 4-cycie, spark-ignited engine.

Exhaust Stroke (Point 4 to Point 1)

With the exhaust valve open, the upward stroke from BDC to TDCcreates a positive pressure within the cylinder, which forces combustionproducts from the cylinder on the "exhaust" stroke

Two-Stroke Cycle Engine

Two-stroke cycle engines require two engine strokes or 360 degrees ofcrankshaft rotation to complete the basic functions of intake, compres-sion, power, and exhaust Figures 16-3 and 16-4 illustrate a cross sectionand a P~V diagram for a two-cycle engine, respectively In this commontype of engine, the piston in its traverse covers and uncovers passages or

out-of high positive pressure to completely expel exhaust gases While a cycle engine has the ability to produce power with each down motion ofthe piston, it is at the expense of some external means of compressingenough air to fill the cylinder and to expel the combustion products fromthe previous cycle ("scavenge" the cylinder) as well.

two-Figure 16-4 Idealized P-V diagram for a 2-cycle engine.

The Compression Stroke

As the piston begins its leftward stroke from bottom dead center(BDC), both inlet and exhaust ports are uncovered and air from someexternal source is flowing through the cylinder Directional control isprovided through port and/or piston design to ensure the most completecylinder scavenging possible

At Point 2, the air intake is closed, but compression does not beginuntil the exhaust port is covered also Shortly after the exhaust port isclosed and compression of the trapped air begins, fuel is injected at Point

3 into the cylinder through a high pressure fuel valve At Point 4, justprior to completion of the compression stroke, a spark ignites the fuel/airmixture and the pressure rises rapidly through the remainder of the com-pression stroke and the beginning of the "power" stroke,