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STEAM TRAPS, EXPANSION JOINT

 

 

Steam Traps:

Steam tarps are automatic relief valves that release condensate from a

steam header. Special Traps also removes air and non-condensate from

the steam space particularly in surface condensers of very big turbines.

Steam traps are mainly divided into three groups:  Thermostatic,

Mechanical and Kinematic. Each of these types contains groups of various

specific styles according to service condition and application.

 

Thermostatic:- Steam traps open and close with the expansion of a

temperature sensitive element in the trap called disc. These traps

operates on the principal of temperature difference between steam,

steam condensate and air. Thermodynamic traps may be balance type

traps, bimetallic traps or pinch type traps. Balance pressure traps open

and close through the expansion and contraction of a temperature

sensitive element that responds to the lower temperature created by the

condensate and non-condensing gases in the trap.Bimetallic traps have a

metal element with the proper thermal co-efficient of expansion for the

application.

Pinch-traps uses self regulating action due to modulator fixed inside. The

pinch modulator begins to automatically close off flow as the wax around

the modulator expands with the passage of hot condensate. As the

condensate builds up and cools the wax around the modulator contracts

allowing the orifice to open and create flow.

 

Mechanical steam traps:-These are buoyancy operated i.e. based on

the difference in density between steam and condensate. These are float

thermostatic traps and bucket traps. Bucket type traps are widely used.

Bucket traps are build around floating buckets that have their open side

facing downwards(Inverted bucket). Steam entering these traps is fed into

the bucket causing it to flow to the top of the surrounding pool of

condensate and close the discharge valve. When only condensate enters

the traps(when more condensate collected the bucket tends to rise due to

buoyancy caused by difference in density) the bucket sinks, opening the

discharge valve.

In case of float traps the trap consists of a floating chamber with a

discharge orifice at the bottom. The float responds to the amount of

condensate and moves up and down. When it rises up it opens the orifice

and allow the condensate to drain out.

 

 

 

Kinematic steam traps:-It is based on the different flow characteristic of

steam and condensate. These are disc steam traps, piston steam traps,

impulse steam traps and orifice traps. Thermodynamic disc steam traps

are very robust with a simple mode of operation by the dynamic effect. It

is widely used in steam tracings. Disc traps consists of a float disc resting

on a smaller circular seat. The disc is enclosed in a chamber above the

seat. On start up incoming pressure raises the disc and cool condensate

and air is discharged from the inner ring under the disc located under the

perimeter of the disc. When hot condensate flow through the inlet

passage into the chamber under the disc drops in pressure and releases

flash steam moving at high velocity a low pressure(injector) area under

the disc drawing it towards disc. Again at the same time flash steam

pressure builds up inside the chamber above the disc forcing it down

against the incoming condensate until it seats on the inner ring. At this

point the flash steam is trapped inside the upper chamber and the

pressure above the disc equals the pressure under the disc. But the force

on the trap is greater as the surface area is more.

When the trapped pressure in the upper chamber falls (as the flash steam

condenses) the disc is raised by the bottom higher pressure and

condensate  is allowed to go out. These cycle repeats.

Traps are very essential in a steam line as the increase of condensate

amount in a steam header decreases the thermal efficiency and also

condensate is very corrosive. So traps are used to drain the condensate

from point to point in the line. Some times condensate causes hammering

in the steam lines which is very dangerous as it may cause mechanical

failure of the lines.

 

Selection of steam traps:- A good steam traps must trap the condensate

and  hold back steam while at the same time not restricting the passage

of condensate, air and other condensable gases to drain out from the

system. The selection of traps depends on system pressure, system

temperature, condensate load, air venting, system design and

maintenance needs.

Dirt and debris content is another major factor when selecting traps. Pipe

dirt created during installation and the products of corrosion also need to

be considered and so a strainer must be provided in the traps in the

upstream of steam line.

Diffuser:- When steam trap drains to atmosphere it is possible to

discharge hot condensate with certain amount of flashing steam which is

at higher pressure relative to the condensate pressure. If diffuser is

installed at the open end pipe of the drain of traps then it will reduce the

velocity of discharge and also sound levels up to 80%.

                                     

THERMODYNAMIC STEAM TRAPS                                        DIFFUSER           

FLOAT THERMOSTATIC TRAP

 

EXPANSION JOINT AND FLOW THROUGH PIPES:

Expansion joint are incorporated in pipe and duct system to take up or

withstand the thermal expansion, pressure variations, vibration and

misalignment to prevent damage to the pipe work system. They are

installed in air, steam and flue gas duct system of all power plant,

chemical industries, gas turbine system, petrochemical industries, Primary

reformer ducts and burners,  steel plants etc. In case of steam lines or any

pipes that carry hot fluids at higher temperature a loop is provided to

compensate expansion of pipe lines. With the increase of temperature

from ambient to working conditions, stress is created in the line due to

thermal expansion. If this expansion is not taken care, it will stress certain

areas within the distribution system e.g. piping joints, which could results

in fracture at certain extremes. The piping work system must be sufficient

flexible to accommodate the movement of the components due to

expansion and contraction. Piping supports are designed keeping in mind

the flexibility of the piping system. Roller supports may be the ideal

method for supporting big pipe lines for allowing the pipe to move in

either direction. The expansion loop are another alternatives to avoid the

movement of the pipe lines with service fluids in. These loops may be full

loop or horse shoe loop or expansion loop depending upon the

requirement.

Full loop:- This is simply one complete turn of the pipe. The full loop does

not produce a force in opposition to the expanding pipe work and there is

a tendency to unwind which produces stress on the end joints.

 

 

 

                                               

 

                                      Full Loop

 

 

Horseshoe loop: In this case pressure does not tends to blow the ends of

the loop apart (loop and the main are on the same plane)but, there is a

slight straightening our effect. if this loop arrangements are vertically

above the pipe then a drain must be provided on the upstream side.

 

 

 

 

 

                                      Horseshoe loop

 

Expansion loop: This is a simple loop fabricated from straight pipes and

elbows and widely used.

 

 

 

 

 

                   Expansion Loop

 

Expansion Bellows:

In low pressure high flow piping, thermal expansion is taken care by

Expansion joints, which may be of bellow type or flexible fiber material.

Expansion joints  needs to have tie-rod to limit the sliding and expansion.

The material of construction of bellow are generally stainless steel but it

can be of special grades of SS or Teflon coated fabrics as per need. Some

expansion joints may have a sleeve inside the bellow of fiber which

basically works as a deflector inside to prevent direct contact between

flue gas and the below. In such case the expansion joints are marked with

direction of flow. These are called uni directional bellows.

 

Allowance for expansion:

Expansion(mm)= L x t x

where

L=length of pipe between anchor in metere(M)

t=temperature difference between ambient and working temp in

°C

= Co-efficient of thermal expansion inmm/m degree C x 1/1000

Co-efficient of thermal expansion of materials can be had from the table

for different temperature range and for different material.

In various system there can be different types of expansion joints which

are used for specific services. Expansion joints can be of single bellow

type, universal expansion joint, Internal sleeve expansion joint, Hinged

expansion joint etc. Genarally all expansion joints are designed and

manufactured under EJMA (Expansion Joints Manufacturing association).

All OEMs supplies special instructions for the erection / maintenance of

expansion joints. These instructions shall be followed strictly.

The bellow is the main flexible component of the expansion joint. It must

be strong enough circumferentially to withstand the pressure and

deflection of the joint. Bellows may be of  one or more convolutions. The

total movement capacity of a bellow is proportional to the number of

convolutions it contains. Bellows are subjected to withstand cyclic

deflection and pressure.

Most bellows fail by circumferential cracking resulting from cyclic bending

stresses or fatigue. The best design is a compromise or balance between

pressure strength and flexibility and so good factor of safety shall be

taken to avoid fatigue failure.

 

There are other components in a expansion bellow joints besides bellow.

e.g. control rods, cover, sleeve, anchor, flanges, limits rod, guide etc. Each

component need care while designing the expansion joint for a particular

service.

 

EJMA standards fifth edition define each of the

components of expansion joint and various type of expansion joint

according to various service and application.