choke valves technical clarification

7/29/2019 Choke Valves Technical Clarification

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Choke Valves Technical Clarification

Common Problems with Choke Valves

Corrosion

Corrosion cases are generally associated with the choice of wrong materials. This is not

always the result of a wrong selection by the vendor. Many companies use an in house

choke valve specification based on history. The result of this approach is so called

company-blindness, as an oil/gas well is a varying supply of medium. One well may

produce Carbon Dioxide, while the other doesn't. A standard specification is always a

threat to vendors as they may find that when non- complying disqualification results

with the customer having the wrong material selected for the application. If Carbon

Dioxide is present, one may want to specify other materials for example, Carbon Steel

complete with 17-4ph stem.

In this case a Carbon Steel Inconel cladded body with an Inconel stem can be a solution.

Erosion

One has to make the difference between single phase and two-phase scenarios. Velocity

can cause body erosion, trim erosion and piping erosion. Although in general there are

different philosophies with respect to what is and what is not acceptable, the conclusion

is, that velocities in general are too high.

For liquid application, the velocity has to be controlled below 5-7 m/s measured in the

outlet of the flange, for two phase this is 10-15 m/s and for gas 25 - 40 m/s

This statement is subject to discussion as it does not include for sand production and it

also does not take the gas/liquid ratio into account for two-phase situations. Furthermore

one has to bear in mind that often choke valves are reduced in size, for example a 4-Inch

nominal body with 6 inch inlet and/or outlet flanges. If the velocity in the outlet flange

is still within the above mentioned rule of thumb it does not automatically indicate that

everything will be expected to work out, as the velocity in the so called nominal body

goes up and may well exceed what is considered to be acceptable.

In other words a choke should be calculated first of all with the same body and flange

size before reducing the nom body size. If the velocity is lower than mentioned in the

above rule of thumb, one can calculate a smaller nominal body with smaller flange. If

still within the given limits a smaller body with larger flanges can be used with reduced

risk of erosion.

Cavitation

Cavitation should not be taken lightly as it can not only lead to erosion, but it can also

cause vibration. The vibration may shatter brittle materials as Tungsten Carbide often

used fortrim bean and plug).


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Many choke designs in today's market are still not fitted with a trim style which ensures

good controllability. Good controllability includes the prevention of Cavitation. This

can onlybe achieved by pressure drop control through the trim.

Based control valve experience the following is concluded:

Cavitation control and good controllability can only be guaranteed by using a multiple

orifice trim. Fluid has to be symmetrically distributed and energy dissipation must take

place in the centre of the valve to prevent body erosion.

Leaking

Leaking is defined as leaking to atmosphere. This is sometimes caused by the selection

of incorrect materials as described previously under corrosion, but also can be attributed

to general design.

Almost all available chokes in today's market have a split body design, these vary froma forged block (main body) with weld on or fitted Flange connections (adapters) to main

body bolted bonnet type.

A one-piece body casting is one answer to these problems.

However, the process fluid has to pass through seals before it can leak to the outside

world. Seals utilised on almost all designs are of the O'Ring type. As a choke is used

under high pressure and pressure drop, O'Rings may be subjected to explosive

decompression. This can cause them to split or deform. Should this happen the sealing

property is lost.

Explosive decompression often occurs when Viton material is used. This happens to be

one of the most suitable resilient materials for hydrocarbon service. Hence, there is a

catch two situation if we wish to utilise Viton as it is subject to explosive

decompression. In addition the service of the 0'rings has to be considered as this can

differ from static to dynamic applications. Most problems with O'rings occur in

dynamic applications.

Problem solving

Corrosion

Corrosion is relatively easily solved, as a choice of suitable materials is widely

available. However a comprehensive knowledge of materials is required as

combinations of certain materials may lead to galling. Also material selection may not

meet the required standards e.g., ASTM A-216 WCB cannot be used for class API

10,000 as it will not meet the strength requirements.

Therefore one should select ASTM A487 Gr 1 class C to comply with API.

Please refer to appendix for typical material selections.

Erosion


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As described previously velocity is the most important design criteria, this in

combination with a well-designed trim will limit any erosion.

Calculating velocities hasto be based on general calculations for gas, liquids and a

combination of the two (two-phase). A calculation is made to determine the required

Cv. This Cv can be achieved by fitting different trims in nominal body sizes.

However different pipe diameter selection produces different velocities. Relatively low

velocities are commercially not advised, as one will procure a too large size choke. Too

high velocities are likely to result in erosion of the body with an associated high

maintenance cost.

Trim selection is of importance. Mokveld chokes are equipped with a cage provided

with holes uniformly distributed over the full circumference. This design ensures that

the fluid is symmetrically distributed. The many flow jets are diametrically opposed.

Consequently, the energy is dissipated in the centre of the valve. This occurs in the fluid

itself and not near the surface of any choke component.

Also, preferential flow, the major cause of body erosion, is fully avoided.

To optimise the resistance against erosion Mokveld have also enlarged the gallery area

in the choke body. This again is of influence with respect to velocity.

Cavitation and control problems

The pressure recovery factor of Mokveld chokes is low as a result of the true cage

concept.. Creating a cage with a few holes or small holes on the first couple of percent

open and then changing to a ring of a few big holes makes no compromise, as this is of

direct influence on the controllability of the process. Furthermoreanon-true cage

design will cause unnecessary wear and tear.

In liquid service (oil production) flashing and high velocities in the downstream piping

is reduced.

The true cage concept allows for high rangeability. Well start-up can be performed at

low flow rates. Also a wide range of well conditions can be controlled without the need

to change the choke trim in future years.

The Mokveld design is pressure balanced. As a result the operating torque is low.

Therefore manually operated chokes do not require intermediate gearboxes. This

improves control sensitivity.

Leaking

The bodies of Mokveld chokes are made of one-piece castings or forgings, this design

eliminates the potential leakage path associated with split-body types.

Pre-loading of the cage and seat (by piston guidance) makes it possible to use primary

metal to metal seats between the cage, the seat and the body. Orings are used in a staticconfiguration and as back-up seals (secondary) only.


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The piston seal is located outside the throttling zone to eliminate the possibility of seal

wear caused by erosion. As the surface of the piston, over which the piston runs, is kept

outside the cage at all choke positions, wear to the piston will not result in failure of the

piston seal.

The design provides tight shut-off by the design of the piston and seat surface. Evenafter lifting the piston of the seat the minimum clearance between cage and piston limits

the flow until the piston reaches the first cage holes. Sealing face washout is therefore

eliminated.

Other features

Complete piston guidance

Rather than having an external sleeve design, the Mokyeld design has its piston fully

guided travelling through the cage. This allows for a cage to be locked in position at top

and bottom.

As the piston is fully guided over the total stroke it also cannot cause the brittle trim

materials to fracture due to vibration- Furthermore it ensures, that all scaling areas are

removed from the throttling area.

One piece body design

More compact when compared to split body design. Also less weight.

Pressure balancing

Due to pressure balancing, the required actuators can be smaller as less thrust is

required. There are also weight and space savings.

Size _and ratings

The Mokveld design is available from 1 inch through to 8 inch with pressure ratings of

ANSI 900 through ANSI 2500 and API 3000 through 10.000.

APPENDIX A:

TYPICAL STD. MATERIAL SELECTION

WL1H -CARBON MEL 1 TUNGSTEN CARBIDEINTERNALS


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01.02 Body : ASTM A216 Gr. WCC01 A3 Bean body ASTM A322 UNS G41300

01.03 Bean insert Tungsten carbide01.05 Guidance ASTM A322 UNS G41300

01 A6 Stem ASTM A564 S1740001.23 Piston ASTM A322 UNS G41300

01.61 Bonnet ASTM A322 UNS G4130001.62 Bonnet nut ASTM A322 UNS G41300

02.01 Piston sleeve Tungsten Carbide39.01 Cage Tungsten Carbide

APPENDIX A:

TYPICAL MATERIAL SELECTION FOR HEAVY EROSIVE/CORROSIVE

SERVICE

01.02 Body- ASTM A216 Gr. WCC01.03 Bean body -ASTM A276 UNS S31803 (Duplex

St.St.)01.03 Bean insert -Tungsten carbide

01,05 Guidance -ASTM A276 UNS S3180301.06 Stem ASTM -B637 N07718 (Inconel)

01.23 Piston ASTM -A276 UNS S3180301.61 Bonnet -UNS S42400 /ASTM A276 UNS S31803

01.62 Bonnet nut -ASTM A276 UNS S31803

02.01 Piston sleeve -Tungsten Carbide39.01 Cage -Tungsten Carbide

choke valves technical clarification

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