Steam Conditioning Manual

8/11/2019 Steam Conditioning Manual

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Bulletin 1- XI


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SUMMARY

INTRODUCTION 2

1 PIPING CONFIGURATION.......................................................................................... 21.1 UPSTREAM PIPING............................................................................................................... 21.2 DOWNSTREAM PIPING.......................................................................................................... 2

1.2.1 Layout .................................................................................................................... 21.2.2 Straight lengths...................................................................................................... 31.2.3 Piping dimension ................................................................................................... 4

1.3 FIX POINTS ........................................................................................................................ 41.4 PIPING FORCES AND TORQUES............................................................................................. 4

2 TEMPERATURE AND PRESSURE SENSORS ......................................................... 52.1 MINIMUM TEMPERATURE...................................................................................................... 52.2 TEMPERATURE SENSOR DISTANCE ....................................................................................... 52.3 PIPING LAYOUT ................................................................................................................... 5

2.4 PRESSURE SENSOR ............................................................................................................ 63 DRAINS........................................................................................................................ 74 PREHEATING.............................................................................................................. 105 PIPING INSIDE PROTECTION.................................................................................... 126 PROTECTION OF THERMOWELL.............................................................................. 137 NOISE .......................................................................................................................... 14

7.1 PREDICTION........................................................................................................................ 147.2 INSULATION ........................................................................................................................ 147.3 OTHER MEASURES TO REDUCE NOISE................................................................................... 14

8 BLOW-OUT.................................................................................................................. 169 CLEANLINESS............................................................................................................. 1710 LIFETIME CONSIDERATIONS................................................................................... 17

Polnadocuments dealing with desuperheating:

NTG 76/ 670 Enthalpic control system

NTG 76/674 Installation of variable area steam desuperheaters

NTG 76/675 1-5800 series control valves with internal injection

NTG 76/680 Acoustical treatment


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INTRODUCTIONRecommendations outlined in this document apply both to integral desuperheating control valves1-5000 and 1-9000 series, but some parts are also applicable to individual Polnadesuperheaters3-4000 series. Should these recommendations conflict with information provided by dimensionaldrawings, the latter will prevail on the content of this document.

1 PIPING CONFIGURATION

1.1 UPSTREAM PIPING

Where two or more conditioning control valves are branched from the same header, somemeasures must be taken to prevent flow instability due to pressure resonance in inletlines.As shown in Fig. 1 the branches are tolerated provided they are designed with differentruns.The straight pipe without disturbances installed upstream the control valve must be

longer than 6 times the inlet diameter of body including reducer , as shown in Fig. 2. Thisrecommendation accounts for noise and vibration aspects more than flow rate prediction.Where elbow is unavoidable close to valve inlet, the minimum straight run upstream thevalve depends on the radius of the elbow, and it should be 6 times DN1 for short radius(1,5DN) and neglected (elbow connected directly to the body) for long radius elbow withR5DN. Interpolation is possible for intermediate Fig.s.

Fig. 1 Layout examples with several desuperheating stations branched from a

common header

1.2 DOWNSTREAM PIPING

1.2.1 Layout

Avoid rising piping arrangement to prevent condensate collection.Also when accurate draining is performed, accumulation of water excess in rising pipemay occur during start-up operations or for water supply malfunctions. Thus,installation of angle body types 1-5700 and 1-5600 with outlet upwards, is notpermitted.


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1.2.2 Straight lengths

To prevent water/steam separation a minimum straight section of 6 meters should beprovided downstream the water injection section (the last one for two sections design)up to the first piping bend. This apply also to variable diameter piping, provided thesteam velocity requirements are fulfilled in any section (see Fig. 2).

note 1 see also subclause 1.1note 2 can be reduced when R is 3 DN2or greater but not below 5 xDN2note 3see also subclause 2.4note 4when two or more injecting sections are provided the last one must be considerednote 5 see subclause 2.2

Fig. 2 Piping arrangement around the desuperheater minimum distances andstraight-lengths

The straight piping section can be conveniently reduced where:

- long radius bends (R3DN) are provided- high efficiency desuperheaters Spraysat 1-4442 or fixed area 3-4500 series are

used

However straight length should never be less than 5 times the inner diameter of

desuperheater outlet connection.Should special applications require longer straight runs they will be specified ontechnical documentation.


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1.2.3 Piping dimension

The pipe installed downstream the desuperheater section must be sized to obtain aminimum velocity and consequently a satisfactory turbulence level of flow.Such requirement is fulfilled when the pipe size is the same of outlet connection ofdesuperheating section which is properly selected by Polna. So, theserecommendations apply to larger downstream piping sections connected to the outlet ofcontrol valve through an expander.In the following table, the minimum steam velocity values are listed for all POLNAdesuperheaters and are calculated on downstream section taking into account flow rateand conditions of steam upstream the water injection.Due to the low required steam velocities the size of downstream piping must bechecked at minimum steam flow rate according to the process turndown.The data listed in the table of Fig. 3 are applicable as well to individual desuperheatingPolnaequipments described in NTG 76/67 document.

Minimum steam velocity [m/s]Type Description

DN 6 DN 8 and 10 DN 12

LVL single nozzle 9 12 14

LVPsingle and double nozzle probe

design8 10 12

LVM multiple nozzle n.a. 10 12

Spraysat multiple nozze control valve n.a. 9 11

LF fixed area probe design 4 5 6

These values of velocity represent the minimum operating limits in the specific range of workingconditions that allow the dragging of the water drops and must be fulfilled independently from the

influence of other process characteristics.Lower values of steam velocity can be allowed for specific application.

Fig. 3- Minimum steam velocity upstream the injection point for different Polnadesuperheaters fitted downstream the main control valve.

1.3 FIX POINTS

- The valve body can not be used as a piping fix point- Also outlet pipe should not be used for connecting piping to frame work but, when this

is unavoidable, measures must be taken to prevent transmission of vibrations andnoise through fixed points.

1.4 PIPING FORCES AND TORQUES

Polna control valves are designed to insure the body housing is stiffer than the pipeconnected to it. Thats means the section modulus of any body section is greater than thatof the pipe either on inlet and outlet connections.As a result of this design rule the valve body is able to transmit stresses without sufferingany unacceptable deformations which might impair the regular operation of valve.When evaluating the section modules, possible differences between materialscharacteristics of valve body and pipe at operating temperature must be considered.Where, for particular reasons, piping has higher section modulus than valve bodybranches, customer must inform Polnaof the forces and torques transmitted by piping

and a check of body resistance must be performed on the basis of these information.


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2 TEMPERATURE AND PRESSURE SENSORS

2.1 MINIMUM TEMPERATURE

The minimum temperature for a correct desuperheating is 10C over the saturation steamtemperature referred to downstream pressure. Some applications are known where a T=6C was successfully processed.The higher the difference Td- Tsthe more efficient the desuperheating process is, withthe same equipments and installation.

2.2 TEMPERATURE SENSOR DISTANCE

The minimum distance of the temperature sensor from the water injection point must not

be lower than the maximum value betweenLmin(1) and Lmin(2).

(1) twtsxVL )1min(

(2) pTLL min)2min(

where:

Lmin(1) : minimum distance between injection point and temperature sensor to allow the watervaporization, taking into account the minimum required time for water heating andevaporation -m

Lmin(2) : minimum distance between injection point and temperature sensor taking into accountthermodynamic kinetics of vaporization and dragging of droplets - m

LminT : uncorrected minimum distance of temperature sensor from injection point as a functionof difference between desuperheated steam temperature and saturated steamtemperature (Td-Ts) - m (see Fig. 4)

V : steam velocity in downstream piping at superheated conditions - m/s

x necessary time to get the full water evaporation. Use x = 0.2 where the p across the

nozzle complies with the minimum recommended values -sTd-Ts : difference between requested final temperature Tdand saturated steam temperature

Tsat downstream conditions - C

Ts-Tw : difference between saturated steam temperature Tsat downstream conditions and theinjected water temperature Tw- C

p : correction factor for steam pressure upstream the desuperheating section (see Fig. 5)

ts-tw : correction factor for water temperature versus the saturated steam temperature atdownstream conditions (see Fig. 6)

To avoid excessive response time of the system, the distance between temperaturesensor and the injection point must be lower than VtMAX, where tMAX (seconds) is themaximum allowable time for a stable process control.

For normal applications tMAX5 seconds is assumed and the maximum distance of thetemperature sensor with steam velocity = 20 m/s is 100 meters accordingly.

2.3 PIPING LAYOUT

straight runs are described in clause 1.2 in piping section running up to temperature sensor branches must be avoidedIsolating valves are not allowed in downstream piping as the rating of outlet section ofcontrol valve is generally lower than the inlet side. Different installations have to bediscussed with Polnabefore asking for an offer. This warning is particularly important forH.P. by-pass desuperheating groups due to the large ratings difference.


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3 DRAINS

Steam pipelines must be water-free and it is essential to provide an accurate draining.The main sources of water inside the piping are the condensate and the non evaporated waterinjected through the desuperheating equipment.Free water in steam piping can cause severe damages to pipelines due to dragging of water at

very high steam velocities.Also, excessive vibrations and noise do easily occur.At least, the measurement of steam temperature is strongly affected by the presence of water.Steam conditioning valves 1-5000 and 1-9000 series need to be protected from the damagecaused by free- water entering the body. Therefore drains are necessary both upstream anddownstream the valve.For some piping arrangements Fig. 7 shows where drains should be located.

note 1 recommended for any orientation of upstream pipingnote 2 mandatory when downstream piping is going up or not self-drainingnote 3 for these arrangements the orientation of the drain connection provided on the valve body

must be agreed with Polnanote 4 mandatory for flow-to-close designs

Fig. 7 Drain arrangements around the steam conditioning control valves


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The drain must be located at the lowest point of pipe around the valve.To guarantee a complete collection of water, the drip-leg (or drainpot) directly welded to thepipe should have a diameter not less than thehalf of pipe diameter.Where process layout is designed with two or more parallel desuperheating groups, eachline must be separately drained, both upstream or downstream the valve as appropriate (seeFig. 8).

Top view of a piping arrangement disposed on a horizontal plane: on the left a poor design with a common drainage device fitted after the joining point on the right the recommended arrangement with independent drain facilities

Fig. 8 Draining of parallel desuperheating groups

Fig. 9 Guide to application of preheating of desuperheating control valve


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Angle body 1-5700 series desuperheating control valve

Straight-way globe type 1-5900seriesdesuperheating control valve


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4 PREHEATING

Preheating of the body of desuperheating control valve is recommended depending on thefollowing considerations: valve location

By installing the valve above the header at a distance not exceeding 5 x DN about , whereDN is the inlet valve diameter, an adequate pre-warming is generally guaranteed andpreheating not necessary.

valve operationThe excessive thermal cycling of inlet section of valve body may occur when valve isnormally operating in the closed position (typical for turbine by-pass applications).If an isolating valve is fitted upstream the control valve (not required for turbineapplications), it must be insured that the isolation valve will be correctly opened foroperational readiness of control valve.

thermal gradientsProvided the condensate is fully removed from the body, preheating is stronglyrecommended when temperature difference between live and saturated steam is over

200C.Fig. 9 is a simple guide to select preheating as a function of live steam conditions.A remark: the continuous condensate draining can be considered as a preheatingprocedure having the scope to insure a body temperature not less than that of saturation.

Should preheating performed, according to above considerations, different ways to perform itcan be selected depending on the type of valve and piping layout.

Here below the most usual solutions:

a) recirculation of main steam (Fig. 10)

An auxiliary line is derived from the live steam line and the steam is drawn through thevalve body and recirculated to the same steam line.To accomplish this arrangement the pressures of the take-off points (a) and (b) of by-passmust be slightly different (minimum 0,5 bar) and this drop could be produced by the pipingitself or by an ad hoc component (C) included with this scope (orifice, strainer ).It is recommended to include a manual throttling valve (d) in the by-pass to control and/orstop the flow of heating steam.

b) by-pass of control valve (Figures 11a and 11b)

The injection point is located in the downstream section of body and the differentialpressure through the nozzle is the same of the main valve.Therefore this solution is recommended for low pressure processes only and it is typical forL.P. by-pass applications where the outlet is connected to the condenser.When used in high pressure processes attention must be paid to the valve(d) whichcontrols the heating flow because of the high involved p both in closed and openposition.The sizing of preheating line and steam flow rate is provided by Polnaand depends on thelayout arrangement and the type of valve.


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Fig. 10 Preheating performed by steam recirculation

Fig. 11aBy-pass arrangement to preheat aglobe-type control valve

Fig. 11b By-pass arrangement topreheat an angle-typecontrol valve


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5 PIPING INSIDE PROTECTION

Water dripping can cause the inside pipe surface to crack due to thermal shock.Unless otherwise specified by the manufacturer of the desuperheater it is recommended toprotect the piping against mechanical damage.The following solutions are usually adopted:

protective inside jacket (applicable for steam pipe diameter over 6 only) appropriate additional thicknessSee Fig. 12 for application range of such solutions.

Where over-thickness is used as a protection, minimum values of 3 mm must be adopted inaddition to design values, already inclusive of corrosion allowance.

The additional thickness value, in the temperature range plotted in Fig. 12, is:

(3) ( )wv

TTs = 625,050175

3

where:

TV : steam temperature - CTW : injected water temperature - Cs : 3 6 mm

The pipe downstream section protected by jacket or additional thickness must not be shorterthan : 250 mm + inside DN.

0

100

200

300

400

500

600

0 50 100 200

JACKET

ADDITIONAL THICKNESS > 3 mm

150

(from equation 3)

ADDITIONAL THICKNESS = 3 mm

TC

water

TC

steam

Fig. 12Protection to piping as a functionof Twater and Tsteam


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6 PROTECTION OF THERMOWELL

The temperature sensor protection is recommended where the distance to the injection pointis close to the minimum value and where considerable water amount is injected(desuperheating water flow rate over 25% of steam flow rate to be desuperheated).

The protection may be performed as shown in Fig. 13 and its purpose is to obviate sensor

wetting by not evaporated water.

About 10 mm

FLOW

Fig. 13Typical design of thermowell protection


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Fig. 15 Downstreamarrangements with

piping enlargement

note 1 only whenspace downstream the

station is limited

note 2 min straightlength according to

requirements of Fig. 2.

note 3 best solution tobe selected when

consistent withallowable space and

min steam velocity (see

subclause 1.2.3)


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8 BLOW-OUT

Unless otherwise specified by mounting instructions, disassembling the valve before weldinginto the piping is not strictly necessary.However the piping must be cleaned upstream and free from any solid particles.Where the piping upstream the valve shall be cleaned by blowout after welding, the valve

must be disassembled by removing internal parts except the seat ring.Blow-through the body can not generally be performed due to the welded-in downstreamsilencers and desuperheating devices. For exceptions to this rule please consult POLNAengineering department.After welding, a special spool must be fitted inside the body as outlined in Fig. 16. The spoolmust be welded to exhausting chimney.Pay attention to the maximum pressure of the steam used for the blow-out , as specified inthe technical documents. Usually, the max allowable pressure of POLNAspools is 30 bar fora temperature not exceeding 400C.The POLNAspool allows for a long steam discharge with no danger for the seating surfacesof valve.

Fig. 16 Blow-out of upstream pipingsection

Blow-out must be carried out after welding of the valve body into the piping.Check the cleaning of inside body cavity before reassembly, especially with non self-draining

body shapes (ex. 1-5900 series)


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9 CLEANLINESS

Throughout assembling operations actuator and valve should be protected to prevent any dirt,dust orforeign matter from entering the valve and actuator.We recommend to treat all parts with an appropriate agent in the event of long dwelling time

between installation and start-up.When hydraulic actuators are provided, the oil connections must be kept carefully closed. Inthis case control units and other electric devices must be installed just before start-upoperations.

10 LIFETIME CONSIDERATIONS

The operating service which this type valves are intended to frequently shows many changesof pressure and temperature, making critical the fatigue resistance of valve body.

Unless otherwise requested these POLNA valves are designed according to TRD 301criteria in order to obtain the following performances:

- maximum cold start-up : 2.000- maximum corresponding combined cycles : 10.000- minimum allowed rate of temperature change : 2 K/min

For extreme design temperatures (above 545C) the 9% Cr steel (A 217 C12A, SA 182 F91) isnormally used for the construction of body and, in this case, the calculation leads to allowabletemperature change rates of 8 K/min or better.

Where longer lifetime is requested, the use of 9% Cr steel, or equivalent, is mandatory to

guarantee an acceptable value of temperature change rate.


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Issue 11/2002 ACA 0593

Steam Conditioning Manual

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