Ask the Expert

The questions this issue relate to the maintenance and servicing of corrosion monitoring devices for pipelines and process pipework, and conflicts between coating specifications and the product data sheet requirements.

In a previous issue (editor, March/April 2020) the question of the selection of suitable equipment for effective corrosion/erosion monitoring in process plants and pipelines operating across a wide range of temperatures and pressures, was discussed.

In order to be in contact with the process fluids to test them, devices such as coupons (normally of same material as the pipe), electrical resistance probes, bio probes, chemical injection quills, galvanic probes and other sampling equipment are inserted through proprietary access fittings, either flanged or welded connections.

Can you explain how these devices are maintained, and serviced, in a safe and cost-effective manner? What precautions should be taken before engaging a vendor to perform this work, and what preparations are required at the worksite? AB

This type of monitoring equipment is manufactured in many countries by a wide range of vendors (in varying degrees of quality and equipment monitoring sensitivity), therefore carefully review, all associated QA documentation, materials of construction, test certification and installation procedures.

Great care should be taken in the selection and purchase of such intrusive equipment to check that it complies with all relevant standards such as NACE MR0175 requirements (Parts 1/2/3 – Cracking resistance of materials) and NACE MR0103 (specifically for sour service in refineries).

An intrusive monitoring, and associated servicing, system normally comprises of 5 main components – high pressure (HP) online retrieval tool, a HP double block and bleed service / safety valve, a mechanical or hydraulic access fitting system (permanently connected to outside of the pipe/pipeline or pressure vessel), the internal monitoring device holder (with seals for primary pressure isolation) and the device itself that is to be serviced, such as a test coupon. In addition, a heavy-duty cap provides secondary pressure sealing/containment in normal operation, after the tool has been removed for service.

It should be noted that these monitoring devices form part of a pressurised system for process containment, and the regular corrosion risk assessment data obtained from them contributes to determining how frequently the remaining piping circuit (which can be of considerable length) requires to be monitored by full inspection and other NDT – non-destructive testing methods such as ultrasonic or radiography. The data from these devices (often now transmitted wirelessly), also influences other mitigation measures such as control of water content and chemical injection, with adjustments made to chemical dosing according to the corrosion rate measured on the test device, to within agreed targets / KPI’s – Key Performance Indicators. Further, these devices also monitor problematic contaminants such as bacteria (through sessile swabs taken at device retrieval) that can accumulate in the system, especially when the system is put out of service for any significant period of time without pre-treatment (e.g. biocide addition).

Removal of these in-service devices should only be undertaken by competent/trained personnel, due to the risks of injury and potential loss of process containment, if not performed in accordance with established practices for safe retrieval. All such measuring equipment would normally be pressure tested and checked as being in good condition prior to shipping to site, this being independently witnessed as compliant with agreed pressure test procedures. Every test fitting at the workplace will have its own unique ID (Tag. No.) and set of risks, therefore as well as utilising the relevant fitting retrieval procedure (either for online or offline servicing) a task risk assessment must be performed to identify all hazards (and required mitigations), specific to the retrieval/service location. For example, the device may be located at height and require scaffold erection for the service team (normally 2 people), or the fitting maybe located to the underside of the pipework (6 o’clock position), in which case, there may be risks of fitting/thread blockages from internal debris or spillage of fluids at device removal (when environmental precautions will be required), especially if it has not been serviced regularly. It is also not recommended to perform the 1st service of any such equipment on a live / online basis.

Before the start of any work, it is necessary to consult with the facility’s owner/client who should have maintained an access fittings register (with the vendor’s assistance), of the operating and service history of each Corrosion Monitoring fitting installed on the facility. Generally speaking, a good and frequent service history will allow regular safe in-service/live retrieval and maintenance activities to be performed, and these previous operations will have already identified the risks and should such information not be forthcoming, or is lacking in detail, then only offline/non-live retrieval should be performed (with hand tools), by persons competent to do so.

It is strongly recommended that reference should be made to the new Energy Institute (EI) Guidelines 1st Edition, for the management of access fittings for pressurised systems, published in August 2020.1

This document covers the lifecycle from the design, application, registration and safe handling of the different types of fittings through to abandonment/removal. The types of degradation which can be expected are defined, as well as methods of monitoring and the maintenance required. For more general guidance on effective corrosion control and prevention, the EI Guidance for corrosion management in oil and gas production and processing, 2nd edition should also be consulted.2
Primary pressure retention/process containment is of the utmost importance to all operators for reasons of personnel safety and good control of environmental and business risks. ST

How should a conflict between the requirements, such as for film thickness, in a product data sheet, and the specification, be resolved? JS

The fast and easy answer is that the detailed requirements (specification) will always take priority over the general. With this I mean, the specification is specific or unique for a project or a certain application, and this would supersede the more general content of a Product Data Sheet (PDS).

A typical PDS in the paint industry consists of:

  • Description and recommended use
    Recommendations on pre-treatment
    Guidelines on application, methods, thinning, mixing, film thickness, 
 overcoating intervals etc.
    Physical data for the product, drying time, solid content, colour(s), 
 Volatile Organic Compounds (VOC), specific gravity etc.

Some PDSs are more detailed than others ranging from one to up to ten pages. A well written PDS presents the framework for the product so that the optimum required product can be chosen and application process be decided upon.

The specification requirements should always be based within the parameters of the PDS framework. If there are deviations from this framework this indicates that the requirements may not be met by the product as it will be used for something which it was not designed to be used for, risking a poor result or at the very least some difficulties in application.
In other words, the specification should always be built within the parameters, directly or indirectly, outlined in the PDS and that is normally how the majority of specifications are developed. But a few do stray outside the framework, some for sound reasons and some for less so.

When data for a PDS is processed it is an integral part of product development. Before a product is released from the R&D lab it will have passed stringent testing and trials to make sure that it meets the product profile, both with regard to performance as a coating and with regard to application properties. The resulting data including the tolerance is recorded into the PDS. Normally these data and tolerances are on what can be considered the conservative or careful side.

Developing or building specifications can be for both a broad range of applications or be more for specific or customised use. The latter is often dealt with by a specialist who can evaluate the solution needed, and design the specification accordingly. For this category it is not uncommon to see deviations from the guiding data in the PDS.

Looking more specifically at deviation from the recommended film thickness as mentioned in the question, how can that be a sound specification? If we assume that the specifier has good insight into the application conditions, (for example they may know that the spraying is undertaken in an automated installation and the control of overthickness is strong) then here there is room for extending the thickness range upwards as the tolerance does not need to be able to absorb the extra thickness that can be expected from manual application.
Similarly, if drying times are considered, the specifier will know if the ventilation is very efficient and there is strong temperature control, meaning matters such as over coating intervals and dry to handle times may be shorter than indicated in the PDS. This can also be applied in reverse of course, if the conditions are poor the times in the specification may be increased.

Simply put there may be good reasons for deviating, but they should always be defendable if challenged and they should be expected to be very close to the values stated in the PDS. Deviations wildly off should not be accepted as they, in principle, constitute a set of questionable data in either document.

As specifications are normally the result of serious work, they should have priority over the product data sheet.

Niels Lund Jensen (Subject Matter Expert, Hempel A/S)

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