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Hydrocarbons Transport Systems: Testing Safety Valves

Disclaimer: This work has been submitted by a student. This is not an example of the work written by our professional academic writers. You can view samples of our professional work here.

Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Published: Wed, 21 Feb 2018

Abstract

In this master thesis the effects of changing the test interval of the land based safety critical valves have been highlighted

Definitions

Production assurance: Also referred to as regularity, is a term used to describe how capable a system is to meet demand for deliveries or performance (Norsok Z-016, 1998).

Availability: The ability of an item to be in a state to perform a required function under given conditions at a given instant of time or during a given time interval assuming that the required external resources are provided.

Production Availability: The ratio of production to planned production, or any other reference level, over a specified period of time (Norsok Z-016, 1998)

Failure: Termination of the ability of an item to perform a required function.

Note 1: After failure the item has a fault.

Note 2: “Failure” is an event, as distinguished from “fault”, which is a state.

Failure mechanism: The physical, chemical or other processes which lead or have led to a failure.

Failure mode: The effect by which a failure is observed on the failed item.

Safety system: A system which realises one or more active safety functions http://www.npd.no/regelverk/r2002/frame_e.htm

Safety functions: Physical measures which reduce the probability of a situation of hazard and accident occurring, or which limit the consequences of an accident. http://www.npd.no/regelverk/r2002/frame_e.htm

Background

Modern production systems are large, complex, automated, and integrated. Failures occur more or less frequently in these complex and large systems. For a production plant, the consequences of failure include high maintenance cost, possible loss of production, and exposure to accidents. It can also lead to annoyance, inconvenience and a lasting customer dissatisfaction that can play havoc with the responsible company’s marketplace position (Croarkin and Tobias, 2007)

So, it is important for the plant engineers and managers to make decisions that can reduce or eliminate the probability of failures or/and their consequences as well as uncertainties in production processes to get better production assurance.

Production Assurance (PA) is introduced by the Norwegian oil and gas industry, which plays a significant role in supporting the decision-making process for managers and engineers dealing with the challenges of meeting various customer requirements as well as production control needs. Therefore, there has recently been a high degree of interest in use of the production assurance concept. (J. Barabady, 2007)

Production assurance (also referred to as regularity) is a term used to describe how capable a system is to meet demand for deliveries or performance (Norsok Z-016, 1998). Production assurance may be quantified by various measures like production availability, throughput capacity, deliverability, or demand availability. The PA concept includes several other concepts, such as reliability, maintainability, availability, and maintenance support performance. Some of these concepts, and their relationships, are illustrated in Figure 1. In the following section, different concepts, of production assurance are briefly reviewed and discussed.

Effective maintenance is necessary to ensure the reliability of plant/equipment. If equipment is unreliable, the profitability of a business can be greatly decreased. Therefore, the benefits of employing the efficient maintenance strategies cannot be underestimated.

Effective equipment maintenance ultimately dictates plant reliability and has great impact on the success and profitability of a Business Unit. There is an increasing industry focus on safety, risk avoidance and environmental awareness, which emphasises the importance of avoiding failure through successful maintenance. As a consequence, maintenance practices often account for an overwhelming percentage of budget expenditure. The financial and safety benefits of employing efficient and effective maintenance strategies for equipment cannot be underestimated.

The Norwegian safety regulations have two kinds of requirements related to maintenance:

  • High level requirements stating that installations, systems and equipment should be maintained in a prudent manner.
  • Detailed and prescriptive requirements for a system or a piece of equipment to be tested or inspected at certain intervals. (The Maintenance Baseline Study; Operation & Maintenance Compendium)
  • Introduction

According to PSAN (Petroleum Safety Authorities Norway) ‘‘Requirements for testing of safety critical valves’’ emphasizes that there should be annual testing of all safety critical valves and intervals for verification have to be established based on; requirements to reliability, knowledge about failure conditions, knowledge about possible consequences from failure conditions, and knowledge about valve characteristics (T.E. Nøkland, H.S. Wiencke, T.Aven ; Identification of safety critical valves – a risk based approach)

In testing of safety critical valves means that production must be shut down, the valve must be closed, pressure downstream the valve is bled off, and pressure build-up is measured.

It has been observed that often these tests are carried out during turnarounds, not influencing production downtime, Even though test are labour intensive, costs related to such test are limited but sometimes the situation is different. Some platforms do not perform turnarounds each year and production may have to be shut down for hours because of these tests. In most cases these shut downs are also affecting other installations. This is of course an expensive operation that the operators want to limit to what is needed to maintain the required safety level; not only because of the loss of production and loss of income, but also because a shut down of the process and manual intervention into the hydrocarbon system has a negative effect on the safety level in it self (PSAN, 2004) ;T. Aven, H.S. Wiencke, T.E Nøkland (2006)

For instance, If we focus on the barrier functions of the valves, and If we prove the same safety level with alternative test procedures or risk reducing measures then we could be able to justify an increase of test intervals of safety critical valves; T. Aven, H.S. Wiencke, T.E Nøkland (2006)

Thesis objective/Problem Statement:

This thesis is a part of RAMONA project which focuses on regularity and deliverability of the Norwegian gas transport system.

In production plants, generally incidents and events occur from both safety-related and technical integrity-related concerns. “Safety integrity related incidents are those endangering harm to people. Working without Personal Protective Equipment (PPE), personal injuries, and fire and explosions are some of the examples that come under safety integrity-related incidents. Technical integrity-related incidents on the other hand, refers to a wide area of technical incidents arising from day to day operations, and those resulting in the possible reduction or loss of daily production’’; see (J. Raza & J.P. Liyanage)

The main objective of this thesis is to ‘‘discuss the effects of changing the test interval of land based safety critical valves in hydrocarbons transport systems’’.

Changing test interval means increase or decrease of the interval period compare to current standard test interval (which is one year) followed by industry.

Working method

Analytical Learning Framework:

Limitations

Among other factors that had influence on this project in terms of delimiting it in some way, can be mentioned available time, available literature and language skills of author.

1.5 Regulations/ Standards:

This chapter is about different Regulations/standards presented by the authority of the Norwegian Petroleum Directorate (NPD) and the Petroleum Safety Authority Norway (PSA) related to maintenance program and further related to safety critical systems.

The legislation consists of a two parts; resource management or ‘‘Resource hierarchic’’ part and a health, environment and safety (HES) or ‘‘HES hierarchic’’ part; which further display different legislation levels.

In the HES area, the Norwegian Pollution Control Authority, the Norwegian Social and Health Directorate and the PSA (former NPD) co-operate on joint, total regulations relating to health, environment and safety on the Norwegian continental shelf. Hence, the HES regulations are issued in pursuance of the Petroleum Act, the Pollution Act, the Product Control Act, the Health Personnel Act, The Patients’ Rights Act, The Communicable Diseases Control Act and Health related and Social Preparedness Act. The regulations are the framework regulations (Royal Decree), the management regulations, the information duty regulations, the facilities regulations and the activities regulations. Guidelines to the regulations have been prepared by: http://www.npd.no/regelverk/r2002/frame_e.htm

Regulations are connected together as shown in figure; Some points related to above figure is explained below.

Acts and Regulations come on the first and second level in hierarchy. Then are the guidelines to regulations for detail explanation and similarly these guidelines showed some specific requirement which is called standards.

->Petroleum Activities Legislation (Acts and Regulations)

For example, Petroleum Activities Act § 9-1 says ‘‘The petroleum activities shall be conducted in such manner as to enable a high level of safety to be maintained and further developed in accordance with the technological development’’

->Guidelines to Regulations

These are guidelines to different regulations relating to management, information duty, facilities and activities under the ‘‘Joint Regulations’’. E.g. OLF (Norwegian Oil Industry Association)g recommended guidelines for the application of IEC 61508 and IEC 61511 in the petroleum activities on the Norwegian Continental Shelf,

->Standards: The guidelines to the regulations often refer to recognized standards as a way to fulfill the functional requirements in the regulations. International Standards like ISO, API, IEC, OLF guidelines, EN and NORSOK standards are often used.

->Industry internal governing documents like ‘‘Testing of safety critical valves in gas/condensate pipeline system’’.

In NORSOK standards Z – 008, maintenance defined as –

“The combination of all technical, administrative and managerial actions, including supervision actions, during life cycle of an item intended to retain it in, or restore it to, a state in which it can perform the required function” (PrEN 13306)

Maintenance includes activities such as monitoring, inspection, testing and repairing. This means, that is all what is required to keep or to get the item or system back into desired operating condition.

According to §7 of The Activities Regulations; the safety functions at all times will be able to provide functions and should be designed so that they can be tested and maintained without impairing the performance of the function.

Similarly under the §32 of the Activities Regulations, it says that ‘’Facilities shall have an emergency shutdown system which is able to prevent situations of hazard and accident from developing and to limit the consequences of accidents, on safety functions. The system shall be able to perform the intended functions independently of other systems’’.

Moreover, the emergency shutdown system shall be designed so that it will go to or remain in a safe condition in the event of a failure which may prevent the functioning of the system.

More specifically, ‘’Emergency shutdown valves shall be installed which are capable of stopping streams of hydrocarbons and chemicals to and from the facility, and which isolate the fire areas on the facility’’

In §44 (maintenance programme) under the Activities Regulations states that the emergency shutdown system should be verified in accordance with the safety integrity levels stipulated on the basis of the IEC 61508 standard and OLF’s Guidelines 070. In addition to that plants which are not included by this standard and these guidelines, the operability should be verified through a full-scale function test at least once each year.

The test should cover all parts of the safety function, including closing of valves. The test should also include measurement of interior leakage through closed valves. Recording of the plant’s or equipment’s functionality in situations where the function is triggered or put to use may replace testing of the plant or the equipment,

The OLF (Norwegian Oil Industry Association) recommended guidelines for the application of IEC 61508 and IEC 61511 in the petroleum activities on the Norwegian Continental Shelf, says that Periodical functional tests shall be conducted using a documented procedure to detect covert faults that prevent the SIS (Safety Instrumented Systems) from operating according to the Safety Requirement Specifications. The entire SIS shall be tested including the sensor(s), the logic solver, and the final element(s) (e.g., shutdown valves, motors). (OLF 070)

In addition, It is recommended to record and analyse activation of SIS functions to include the activation as part of the functional testing. If proper operation and documentation thereof exist for a period, the manual proof test for that period may be omitted. Observe that the spurious activation of an ESV due to a PSD, does not test the entire function of the same valve during an ESD action.

Moreover, In OLF guidelines it is mentioned that, some periodic interval (determined by the user), the frequency(s) of testing for the SIS or portions of the SIS shall be re-evaluated based on historical data, installation experience, hardware degradation, software reliability, etc. Change of interval is handled as a modification. Any change to the application logic requires full functional testing, and shall be treated as a modification. Exceptions to this are allowed if appropriate review and partial testing of changes are done to ensure that the SIL has not been compromised.

3. Basics of valves

Valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid. Valves can be handle either liquid or gas applications; Philip L. Skousen (2004)

Valves are used in pipeline systems to control the flow rate, the pressure, or the flow direction of a fluid. They can turn on, turn off, regulate, modulate or isolate the fluid.

3.1 Valve Types

3.1.1 Gate valves:

Gate valves are designed to operate fully open or fully closed; when fully opened, there is very little pressure drop across a gate valve, and when fully closed there is good sealing against pressure.

With the proper mating of a disk to the seat ring, very little or no leakage occurs across the disk when the gate valve is closed. However, some leakage may occur under very low back pressures. Another positive feature of gate valves is that they usually open or close slowly, which prevents fluid hammer and subsequent damage to the piping system.

The main limitation of gate valves is that they are not suitable for throttling applications. When gate valves are used in throttling applications, the flow tends to have high speeds near the gate seat, which leads to erosion. Also, in the partially open state, the valve is prone to vibrate, which can lead to damage. In general gate valves are more subject to seat and disk wear than globe valves, and repairs, such as lapping and grinding, are more difficult to accomplish.

3.1.2 Ball valves:

This rotational-motion valve uses a ball-shaped disk with a hole bored through to stop or start fluid flow. When the valve handle is turned to the open position, the ball is rotated so that the hole lines up with the valve body’s inlet and outlet. When the ball is rotated so the hole is perpendicular to flow, the valve is closed.

Advantage of ball valve is ease of operation, high flow capacity, and a high pressure and temperature tolerance. In addition, they have the ability to provide fire-safe protection, and they can handle severe service chemicals. Ball valves typically have lower cost and weight, and provide tight shutoff and low stem leakage. They can be adapted to for use in multiple port configurations.

3.1.3 Check valves:

The purpose of a check valve is to allow fluid flow in one preferred direction and to prevent back flow or flow in the opposite direction. Ideally, a check valve will begin to close as the pressure drops in a pipeline and the fluid momentum slows. When the flow direction reverses, the check valve should close completely. Check valves can be of the following types: swing, lift and tilting disk.

3.2 Why Testing of Valves/equipment:

In NORSOK standards Z – 008, maintenance defined as –

“a combination of all technical, administrative and managerial actions, including supervision actions, during life cycle of an item intended to retain it in, or restore it to, a state in which it can perform the required function” (PrEN 13306)

According to above definition, that is all what is required to keep or to get the item or system back into desired operating condition.

In §7 of the Activities Regulations it is stated that Facilities shall be equipped with necessary safety functions which at all times are able to:

a)  Detect abnormal conditions,

b)  Prevent abnormal conditions from developing into situations of hazard and accident,

c)  Limit harm in the event of accidents.

Similarly under the §32 of the Activities Regulations, it says that ‘’Facilities shall have an emergency shutdown system which is able to prevent situations of hazard and accident from developing and to limit the consequences of accidents, on safety functions. The system shall be able to perform the intended functions independently of other systems’’

More specifically, ‘’Emergency shutdown valves shall be installed which are capable of stopping streams of hydrocarbons and chemicals to and from the facility, and which isolate the fire areas on the facility’’

No more than a few decades ago, maintenance function was considered as an unwanted necessity, which is almost impossible to manage. This vision changed with time and maintenance became a separate service that had the centre attention on technical aspects, with the weight on specialization and efficient working methods. More recently, the progress was the realization that there were more efficient ways in terms of optimizing use of the means and more effective ways in terms of achieving the desired results and it was positive cooperation with other operating functions (Internal partnership). (Compendium op&maint. Page 2)

In IAEA-TECDOC-1200 is stated that the purposes of monitoring, testing and other preventive maintenance actions are the detection of the degradation and prevention from the failure of the safety functions of systems and equipment and the assurance of prompt correction and restoration of these safety functions.

PRA/PSA can be used in order to optimize the level of inspection and maintenance activities correspondingly to them and risk.

To evaluate ageing effects of an equipment

Check corrosion

To prevent accidental events and damage

To analyse dynamic degradation and failure mechanism.

To estimate the probabilities of degradation.

To access the consequences of different degradation cases and evaluate their severity according to the probabilities of the worst consequences due degradation.

To perform the risk ranking for each component.

To make appropriate recommendations, based on results in order to improve the operation and maintenance.

To keep regularity flow constant , we need to test valves and other equipment periodically.

To check the reliability and availability of the equipement.

From (Working Document, governing doc.)

3.3 Safety Critical valves:

The emergency shut down system (ESD system) is a safety system that constitutes an important barrier (the ESD barrier). Fundamental tasks for the ESD barrier are to stop streams of hydrocarbons and chemicals to and from the facility, and isolate the fire areas on the facility. To manage to do this the ESD barrier are depending by the functionality of ESD valves. (Sverre Viland, 2004; Identifying Safety Critical valves – A Risk Based Approach)

Based on current industry practice, to define whether or not a valve is safety-critical is determined on an evaluation of the safety importance, i.e. how important it is for safety point of view. Therefore an analysis/assessment is needed to demonstrate how the risk level could be affected to the following failure modes:

  • Valve fails to close on demand
  • Valve fails to close within the specified time
  • That it leaks

To identify safety critical valves; the required analysis/assessment is performed in to three steps:

1 – To Identify and illustrate the functions of the valve

Valve functions that are important to safety are identified, i.e. the functions whose failure could result in an unacceptable risk, e.g. failure to close, leakage through closed valve.

A safety critical valve normally has more than one function, these are as follows:

  • Does it have an ESD or PSD function?
  • Is the valve part of an overpressure protection system?
  • Is it designed to close/seal off the flow in both directions?
  • Is the valve part of a double block and bleed setup?
  • Other functions.

2 – To explain the effects on safety of the above failure modes

.3 – To Classify critical/unacceptable leakage rate through the valve

In the onshore plants, acceptable leakage rates generally set higher than for an offshore installation, the main reason for this is due to lower human risk exposure in onshore plants.

The acceptance criteria is determined on the basis of whether the contribution to risk of a leakage through the valve is acceptable, required some measures or not acceptable. According to the performed analysis of some onshore terminals and gas transportation systems, recommended reference values for leakage rates are established in table:

Leak rate [kg/s]

Action

< 0.05

Acceptable

0.05 – 1.0

Perform specific evaluations, Plan for repair.

> 1.0

Not acceptable – repair

Table: Acceptance criteria for leakage through closed valves

The wide range between the lower and upper limits, i.e. from 0.05 kg/s to 1.0 kg/s, is calculated and mainly based on practical considerations. Current industry experience shows that most valves (>99 of 100) satisfy the lower limit requirement i.e. <0.05 kg/s.

3.3.1 Testing Methods:

Testing of safety critical valves can be testing of function (close) or testing of leakage (including interior leakage or leakage through closed valve). The various testing methods are different with respect to the required performance in real shut-down situations.

  • Testing of the function (close) with real shut-down case
  • Testing of the function (close) with plant shut down

This test is not considered complete since the forces acting on the valve body and valve internals are different from the real case. Thus the test does not disclose all relevant failure mechanisms.

  • Partial stroke testing

The main advantage with this test is that one can avoid shut-down of the plant, therefore it is only relevant while the plant it normal operation; but this test is not considered complete because the test does not demonstrate full closure of the valve. Thus the test does not disclose all relevant failure mechanisms.

It is preferred that, a test should reflect the intended function in a real situation. According to industry practice; for an emergency shutdown (ESD) valve, this sort of testing should normally be complete closing of the valve with the system under pressure and in operation.

However, in some cases there may occur unwanted effects of these ideal tests, like economic consequences related to lost production, but also sometimes negative effects on safety and environment.

Based on the industry experience, the optimal system for testing therefore may well be one that applies different test methods, and combinations of tests, in a consistent program, individually tailored to the specific safety critical valve.

Testing methods of leakage through valve

Different testing methods are used to observe the leakage through the safety critical valve:

  • Leakage test through closed valve with full pressure differential across the valve.
  • Leakage test through closed valve with different pressure levels up- and downstream of the valve
  • Leakage test through closed valve, by measurement of leak rates into the valve body/cavity.
  • Leakage test with valve in open position

When we talk about testing of leakage rate through a closed valve; acceptance criteria for leakage rates through the valve at normal full differential pressure across the valve should be defined.

4.1 What are the affects of changing the test interval of safety critical valves?

Changing test interval means increase or decrease of the interval period compare to current standard test interval (which is one year) followed by industry. In usual practical applications testing and inspection is the most relevant and effective means of deterioration control.

The observed failure frequency, together with a criticality evaluation, will be a basis for prioritizing the maintenance work and optimization of test intervals; Aven and vinnem (2007)

In fact cost, the level of risk and the benefits from risk control are closely linked see fig.

We can say increase in benefit from a decision may increase the risk if cost are kept constant or any reduction in risk may reduce the benefits as cost may increase.

  • Test interval for test of function close
  • Test interval of leakage in valve

Test interval> 1 year

Test interval < 1 year

Positive effects

Negative effects

Positive effects

Negative effects

  • Save economic cost
  • Reduction in maintenance cost
  • Avoidance of production loss
  • Less number of process shut downs
  • May cause higher risk related to safety level
  • Performance issues
  • May cause higher frequency of occurrence of failure
  • High reliability and functionality of equipment
  • Improved safety level
  • Higher maintainability and availability
  • May increase leakage
  • Maintenance cost increased
  • More production shut downs may affects other installations
  • Labour intensive

Table 1: Different dilemmas of changing test interval of Safety Critical Valve

4.2.1 Discussion:

There are some advantages and disadvantages related to each dilemma; see table 1.

Firstly, we see that current industry practice about testing of safety critical valve which is once a year; is quiet satisfactory. In the Gassco document TEKD-PR-021/5/; is mentioned about safety critical valve that: ’’The reference value for test interval is 1 year. The program may deviate from this, provided that adequate and documented grounds for this are stated’’

There are many critical factors involve in each dilemma.

Followings are the some ‘‘critical factors and their impacts’’ involved in changing the test interval of ESV. Table 2:

Critical Factors

Impacts

Interval <1 year

Interval =1 year

Interval >1 year

Failure Probability

Very Low

Low

Relatively high

Reliability