Valve Magazine - Spring 2011

that the CCT and CPT of stainless steels will increase with their PREs, which is directly related to their alloying content.

For gasket materials, lower strength, pitting and crevice corrosion resistance are issues that require consideration for materials with high PRE. In many cases for subsea application where the ring groove is overlaid with a CRA, the use of UNS N08825 is marginal. The material does not have sufficient corrosion resistance and is susceptible to corrosion if untreated seawater is used. In such applications, higher alloy materials should be considered, such as UNS N06625.

Fasteners

Material choices for subsea bolting are limited. Historically, bolts were manufactured from either ASTM A193 B7 or ASTM A320 L7. In both cases, the materials are identical and within the composition of low alloy AISI 4140- 4142. The only difference is the impact requirements where A320 is specified for low temperatures. For subsea applications, several options can be used for the protection of alloy steel. These include the use of coatings for corrosion protection in seawater or the use of a CP. With the use of coatings, intimate contact with the CP system is not so important, providing the flange connections are also low-alloy materials. A properly applied coating that is developed for use in seawater should be adequate. In cases, however, where a coating is not used, it is important the CP is effective over the full length of the bolt. Each bolt must be connected to the CP system.

CHEMICAL TREATMENT
AND INJECTION
During operation, all subsea equipment
at one time or another comes in contact
with injected chemicals. Although the
number of materials used for wellhead
applications is limited, the number of
injected chemicals is numerous. Injected
chemicals can include paraffin inhibitors,
asphaltene dispersant, asphaltene sol-
vent, scale inhibitors, corrosion
inhibitors, biocides and combination
products. These chemicals are used to
control corrosion, hydrates and wax for-
mation. They all have proprietary addi-
tives, which chemical suppliers add as a
selling point for their products. The
chemicals are sold to an end user for spe-
cific applications. This is an area in sub-
sea applications that requires close coop-
eration between end users, equipment
manufacturers and chemical suppliers. In
the development of these chemicals, test-
ing should be performed by the chemical
supplier to ensure their chemicals will
have no adverse effects on the materials
with which they may come into contact.

Table 2. Fluid Compatibility Part: Tubing head body Alloy: Alloy steel, stainless or CRA

Paraffin inhibitor Asphaltene dispersant Scale inhibitor Asphaltene solvent Hydrate inhibitor Control fluids Production fluids Annulus fluids

Compatible

No Flow back Yes None None None None Yes

Exposed

N/A
Okay
Yes
N/A
N/A
N/A
N/A
Okay
Such a table is made for each appro-
priate component. Testing by the chemical
supplier can be performed accordingly.

WEAR AND GALLING-
RESISTANT COATINGS

Wear and galling are issues for gate valves as well as any other component where materials come in contact with each other. The problem is compounded with subsea gate valves as a majority of the components are fabricated from stainless steel, which are prone to galling. For gates and seats, spray coatings have proven to provide the necessary galling resistance.