Valve

Valves

A valve is a mechanical device that controls the flow of fluid and pressure within a system or

process. A valve controls system or process fluid flow and pressure by performing any of the

following functions:

·        Stopping and starting fluid flow

·        Varying (throttling) the amount of fluid flow

·        Controlling the direction of fluid flow

·        Regulating downstream system or process pressure

·        Relieving component or piping over pressure

There are many valve designs and types that satisfy one or more of the functions identified

above. A multitude of valve types and designs safely accommodate a wide variety of industrial

applications.

Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal

elements), actuator, and packing. The basic parts of a valve are illustrated in Figure.

Body:

The valve's body is the outer casing of most or the entire valve that contains the internal parts or trim. The bonnet is the part of the encasing through which the stem (see below) passes and that forms a guide and seal for the stem. The bonnet typically screws into or is bolted to the valve body.

PSV valves

Introduction
Pressure Safety Valve (PSV) is one of safety devices in oil and gas production facility, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressure higher than their design pressure. Therefore, the selection of PSV to be installed must be conducted in a careful and proper manner.

These are the questions worth to be asked when you are going to specify details of PSV. 

·        What type of PSV we will have for our process requirements?

·        Is there any easier way for PSV sizing (PSV calculation) rather than calculate it manually?

·        What kind of material shall be chosen for our process requirements?

Prior to the PSV selection, it would be better if we know how the PSV works which will lead us in understanding of critical parts of PSV. Then, the PSV selection process can be done with awareness of some strong points.

Pressure Safety Valve by definition
Cited from API 520 part 1 (Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; Sizing and Selection) about Safety Valve definition: “A safety valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids.” Figure 1 shows Conventional PSV, which is purposed for description only.

 

Figure 1. Conventional Pressure Safety Valve (Taken from API 520 part 1)

How does it work?

Figure 2. Sketch of Pressure Relief Valve

How does the PSV work? Figure 2 is a simple sketch of pressure relief valve which shows the disc held in the closed position by the spring. When system pressure reaches the desired opening pressure, the pressure force of the process fluid pass through the inlet and then it is acting over Area A₁ equals the force of the spring, and the disc will lift and allow fluid to flow out through the outlet. When pressure in the system returns to a safe level, the valve will return to the closed position.

Certain area of the disc and nozzle will allow certain amount of the gas/liquid volume. The area of the nozzle (so called as “Orifice”) needs to be calculated in order to have proper amount flow of the process fluid. This certain area has been standardized in API 526 (Flange Steel Pressure Relief Valves) and designated into certain alphabetic as shown on Table 1.

Since PSV will most likely to be in closed position, it is a good idea to choose some kind of “seal” between disc and the nozzle to keep the process fluid from leaking to the outlet of the PSV.

Conventional, Bellows or Pilot type?

Backpressure considerations
Types of PSV are created due to existence of backpressure. The effect of backpressure can be depicted by Figure 3 which incorporate forces from spring (F_s), process fluid from the pressurized system (P_VA_N), and backpressure (P_BA_N). The P_V is the pressure due to the changes over the pressurized system, and the P_B is the pressure which exist in the outlet of the PSV, we recognize this as a back pressure. As you may see, that the spring – denotes with the F_s – is having main contribution to the force balance, and have a positive direction along the P_B. The overpressure in the pressurized system will increase the magnitude of the P_V, and eventually it will affect the balance of the pressure force, and hence the sum of the P_BA_N and the F_s will be less than the P_VA_N. The spring, which holds the disk and isolates the pressurized system into the outlet of the PSV, is moving upward and the disk will not contain the pressurized system anymore.

Figure 3. Effect of Backpressure to the set pressure (Taken from API 520 part 1)

An extreme example, in the closed position, if backpressure is high enough to compensate the force pressure of process fluid, the force resultant will be zero, in other words the PSV will remain close. In this condition, the PSV is not successfully to fulfill its function. We will examine types of PSV.

Conventional type
This type of PSV is the simplest one as you may see on Figure 4. Usually, this type of PSV is used whenever the existence of back pressure is relatively small (less than 10% of set pressure), or nearly zero. Due to its low immunity to back pressure, the conventional type outlet is vented into atmospheric, and mostly, the fluid to be vented is non-hazardous fluid i.e.: water steam.

Figure 4. Conventional Pressure Safety Valve (Taken from API 520 part 1)

Bellows type
PSV with bellows type or balanced-bellows type is used when the backpressure does not exceed than 50% of set pressure. This type of PSV is almost the same with the conventional ones, but there is additional bellows in it as you may see on Figure 5. The bellows itself has a function to reduce the effect of the backpressure force (P_BA_N) over the disk as you may clearly see on the forces diagram on Figure 3. The bellows contained the upper side of the disc and the rod which connected to the spring from pressure force of process fluid/pressurized system – in which connected through PSV outlet – and the inside chamber of the bellow will be vented to the atmospheric, which obviously has constant pressure. Commonly, this type of PSV does not have a wide range of PSV, hence, it is not so flexible in alteration of set pressure.

Figure 5. Bellows Pressure Relief Valve (Taken from API 520 part 1)

Pilot type
A pilot-operated pressure safety valve consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot as shown on Fig.6. The piston is designed to have a larger area on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than 90% of MAWP

The pilot type has a sensing line and its function is transmitting the built-up pressure that may exist in the pressurized system to the pilot valve. As the pressure in the pressurized system is increasing and reaching the set pressure, the pilot valve will actuate the PSV spring inside the main valve to pop up the PSV. Due to the actuator has no direct contact with the venting system the valve will not relatively be affected by backpressure. Moreover, this type of PSV has a wide range of spring setting, it will be an advantage if we want to change the set pressure on a wide range alternatives.

Figure 6. Typical pilot-operated valve

Multiphase Fluid
How about if we need to release multiphase fluid? Is there another type of PSV which is able to handle that kind of case? Well, it is good question actually. If we are using conventional PSV, we will have big problem in the backpressure consideration if we do have large backpressure or even a variation of backpressure.

Another option is pilot. It also has a week point which is critical on multiphase handling since there will be possibilities that the sensing line will be plugged with non-clean fluid. None will guarantee whether or not the process fluid is “clean” (containing of liquid and gas only). They may have little solids or debris which eventually plug the sensing line.

The last option is the bellows type, since it is relatively unaffected by the backpressure and it has no sensing line like the pilot type has. We will choose this last option, because we only have three available type in the market. It is obvious now that every possible case is not available in ready-on-stock PSV type, we have to conduct an engineering judgment on any possible case within available type.

For comprehensive understanding between types of PSV, Table 2 is describing the advantages and disadvantages each one of them.

What are required for PSV Sizing?

After we have selected the type of the PSV, we should calculate the size of the orifice. Of course this is one of the important step to select PSV. Why do we have to calculate the PSV anyway? If you don’t calculate your PSV, you’re not really sure whether the size is adequate or not to handle the fluid relief. The main principle of PSV sizing: it is fit for purpose. Smaller size of PSV means smaller capacity of the valve and also, bigger size of PSV means bigger capacity of the valve.

The application of the smaller capacity of PSV than its design capacity shall be avoided. Because if the PSV is unable to allow the process fluid to be released, then the pressure in pressurized system is tending to increase and adjacent parts of the pressurized system will be burst or rupture. In other words, the PSV is unable to fulfill its main function.

It is almost similar to the application of bigger capacity of PSV than its design capacity. The bigger capacity from its design capacity means PSV is allowing the process fluid “too much”. If we have pressurized system to be in overpressure condition, the set pressure of the PSV is reached and the process fluids will be vented through the outlet. Due to its large capacity, the pressure in the pressurize system will be decreased rapidly and then the PSV will re-close. But, as the PSV is closing, the pressure in the pressurized system is increasing again and the set pressure of the PSV is reached again, and the PSV will open again. This is what people called as “chattering”, and most of cases the chattering itself is more like to be a rapid vibration. This is an example of bad sizing of PSV because the PSV will be damaged after a chattering. In other words, the PSV is unable to fulfill its main function again.

As a basic of PSV sizing, these following process data as shown on Table 3 shall be provided to calculate the orifice designation.

Table 3. Process Data for PSV Sizing

PSV Sizing using Software

Is there any chance that we can size PSV easier? The answer is “yes”. But you must be careful then, wise people said that: “it’s not about the gun, it’s about the man behind the gun”. Software is only calculating what is coming through it, and do what we told. In another word: garbage in, garbage out.

You can use specific software, which made special for it. The useful software tool for PSV sizing I ever had is Instrucalc Version 5.1, the user interface is as shown in Fig.7. I will use Instrucalc Version 5.1 as description-purposed only, even there are other software which have the same capability.

Figure 7. Instrucalc version 5.1 for PSV sizing.

This software is non-vendor oriented, since its calculation relied on API-520 and ASME Sect.VIII, and almost all vendors are taking reference to those two standards. Instrucalc is best on describing the size of orifice designation, inlet and outlet size and maximum capacity of the valve could handle. Moreover, for Gas Relief and Liquid Relief case, the calculation result of Instrucalc and vendor software is most likely to be the same, that would be a reason for choosing Instrucalc as a general calculation software.

However, for some specific types of PSV from certain vendor, I would rather choose vendor software which is able to calculate various outputs based on their PSV models, especially when reviewing vendor’s proposal. For an instance, Instrucalc will generate certain size of inlet and outlet, which any vendor does not have that size of inlet/outlet. If there is discrepancy with Instrucalc, it doesn’t mean that vendor calculate incorrectly, they just don’t have that size, as Instrucalc has calculated. As long as the size and liquid/gas capacity from vendor proposal is adequate with our technical data, that would be all good.

For some reasons, certain vendor is not allowing their software to be installed side by side with other vendor’s software in a computer. This is a difficult problem since the software’s bugs were intentionally “created” by vendor, which eventually we cannot fix. In case you’re facing this problem, consult your vendor representative for more assistance.

Proper material for parts

Compatibility with the process fluid is achieved by careful selection of materials of construction. Materials must be chosen with sufficient strength to withstand the pressure and temperature of the system fluid. Materials must also resist chemical attack by the process fluid and the local environment to ensure valve function is not impaired over long periods of exposure. The ability to achieve a fine finish on the seating surfaces of the disc and nozzle is required for tight shut off. Rates of expansion caused by temperature of mating parts is another design factor.

Comparison among Vendors

We have some basic knowledge about basic of PSV selection, let’s do some real job here.

Correctness of calculation
We require to pay attention for process data. Mostly, they are root cause of incorrect calculations, wrong data will lead you to some confusing results, so be careful then. Having the process data correctly, we need to see the result and compare them (vendor’s and ours), are they different badly? We need to see, whether the discrepancies are critical or not. As example, the calculation of orifice area from each vendor can be different with the same process data and method of calculation (API-520), but you must pay attention that vendors will refer to the same orifice designation. The same way if vendors offer 1.5 inch of inlet size, while according to our own calculation we need 2 inch. That would be fine if the valve capacity is capable to handle our data process with the size of inlet/outlet pipe is not too large or too small compared to our own calculation.

Material
Material is another important issue since we need the PSV to be “seated” for some years and most probable to handle “bad” fluid process characteristics.

The most critical parts are the spring, seat and disc. We need to pay attention on their material to be proposed by your vendors. The internal part of the PSV is shown in Figure 8.

Figure 8. Internal part of the PSV (Taken from API 520 Part 1)

Spring’s material is one of the important consideration, since it is “muscle” of the PSV. There are many alternatives for the spring’s material, i.e : chrome steel, inconel. Different material will be impacted to the overall price, you should select the material properly.

Seating surface – or seat for short – has a function to contain the pressurized system and the vented system, since it is “clutching” the disc. Usually, we have a soft seated and hard seated options. The hard seated means that it is made from the metal material, i.e : steel. While the soft seated means that it is made from the non-metal material, i.e : kalrez, viton. The advantage of having soft seated that it will have a good isolation, because it is “softer” than the hard seated, so its shape is more flexible to clutch the disk, which the disk is commonly made of stainless steel.
The most exposed part to the process fluid is the disk. That would be a reason that we have to choose a good material of it. Usually the disk is made of stainless steel because of its properties to be able stand on the harsh environment.

Price criteria
In most cases, money talks. High price means high quality, low price means low quality, but you should remember, it is not always true. You shouldn’t believe, for instance, with the low price of the PSV also will has low quality, either with the high price. There must be some overheads over the price components or even low quality of the materials. You should examine vendor’s proposal very carefully and thoroughly, you must go into as detail as possible. If you have any doubt about some points, you must ask to vendor for explanations until you have satisfaction on the answers and you have confident to determine whether or not you are going to accept vendor’s proposal.

What is flange?

What is flange?

A flange is a method of connecting pipes, valves, pumps and other equipment to form a piping system. It also provides easy access for cleaning, inspection or modification. Flanges are usually welded or screwed. Flanged joints are made by bolting together two flanges with a gasket between them to provide a seal.

 

Types of Flanges

·        Welding Neck Flange

Weld neck pipe flanges attach to the pipe by welding the pipe to the neck of the pipe flange. It allows for the transfer of stress from the weld neck pipe flanges to the pipe itself. This also reduces high stress concentration at the base of the hub of the weld neck pipe flanges. Weld neck pipe flanges are often used for high pressure applications. The inside diameter of a weld neck pipe flange is machined to match the inside diameter of the pipe. Weld Neck Pipe flanges with a hub have published specifications that range from 1/2" thru 96". Weld neck pipe flanges are typically provided with a raised face, flat face, or RTJ facing. When a raised face is necessary for weld neck pipe flanges, the standard height is 1/16" for weld neck pipe flanges fewer than 400#. For weld neck pipe flanges of 400# and up, the standard weld neck pipe flange raised face height is 1/4".

 

·        Slip On Flange

These are type of flanges that slide over the end of piping and then welded in place. These flanges are ideal for lower pressure applications. These are easily fitted and welded into different pipes. Welding reduces fabrication costs of these pipes.

·        Socket Weld Flange

Socket welding flange, a popular type of pipe flange, was initially developed for use on small-sized high- pressure piping. The fabrication of this type of flange is similar to that of a slip-on flange. However, the internal pocket of a socket weld flange allows for a smooth bore and better fluid flow. When provided with an internal weld, the static strength of this flange is equal to slip-on flange, but the fatigue strength is 50% greater than double welding slip-on flanges. Smooth bore conditions in such a flange can easily be attained without having to bevel the flange face and, after welding, to reface the socket weld flange as would be required with slip-on flanges. For this reason, the internally welded flange is popular in chemical process piping.

·        Lap Joint Flange

The lapped flanges are widely used in steel plants, sugar mills & distilleries, pumps and petrochemicals, cement and construction industries.

·        Threaded Flange

They are the special type of pipe flanges which can be attached to the pipe without welding. Threaded flanges are threaded in the bore which match an external thread on the pipe. These threads are tapered in order to create a seal between the threaded flange and the pipe as the tapers approach the same diameter.

 

·        Blind Flange

A blind flange is used to close ends of piping systems. It is a kind of round plate with no center hold but with all the proper bolt holes.

·        Purpose of Orifice Flanges

     Orifice Flanges are used with orifice plate or flow nozzle for the purpose of measuring the flow rate of either liquids or gases in the respective pipeline. Orifice flanges generally come with either raised faces or RTJ (Ring Type Joint) facings.

 

I have worked in oil and gas for 4 years and I decided to share my information with others, maybe it helps somebody. I will write about piping, oil and gas equipments.

What is piping?

Within industry, piping is a system of pipes used to convey fluids (liquids and gases) from one location to another. The engineering discipline of piping design studies the efficient transport of fluid.

Industrial process piping (and accompanying in-line components) can be manufactured from wood, fiberglass, glass, steel, aluminum, plastic, copper, and concrete. The in-line components, known as fittings, valves, and other devices, typically sense and control the pressure, flow rate and temperature of the transmitted fluid, and usually are included in the field of Piping Design (or Piping Engineering). Piping systems are documented in piping and instrumentation diagrams (P&IDs). If necessary, pipes can be cleaned by the tube cleaning process.

"Piping" sometimes refers to Piping Design, the detailed specification of the physical piping layout within a process plant or commercial building. In earlier days, this was sometimes called Drafting, Technical drawing, Engineering Drawing, and Design but is today commonly performed by Designers who have learned to use automated Computer Aided Drawing / Computer Aided Design (CAD) software.

Plumbing is a piping system that most people are familiar with, as it constitutes the form of fluid transportation that is used to provide potable water and fuels to their homes and business. Plumbing pipes also remove waste in the form of sewage, and allow venting of sewage gases to the outdoors. Fire sprinkler systems also use piping, and may transport nonpotable or potable water, or other fire-suppression fluids.

Piping also has many other industrial applications, which are crucial for moving raw and semi-processed fluids for refining into more useful products. Some of the more exotic materials of construction are titanium, chrome-moly and various other steel alloys.

Pipes:

Based on the NPS (Nominal Pipe Size) and schedule(wall thickness) of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and B36.19M. For example, NPS 14 Sch 40 has an OD of 14 inches and a wall thickness of 0.437 inches. However the NPS and OD values are not always equal, which can create confusion.

• For NPS ⅛ to 12 inches, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size is always the actual OD.)
• For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD.

The reason for the discrepancy for NPS ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.

For a given NPS, the OD stays fixed and the wall thickness increases with schedule. For a given schedule, the OD increases with NPS while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.

Some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS), although these actually belong to an older system called iron pipe size (IPS). The IPS number is the same as the NPS number. STD is identical to SCH 40S, and 40S is identical to 40 for NPS 1/8 to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive. XXS wall is thicker than schedule 160 from NPS 1/8" to NPS 6" inclusive, and schedule 160 is thicker than XXS wall for NPS 8" and larger.

The "S" designation, for example "NPS Sch 10S", most often indicates stainless steel pipes. However some stainless steel pipes are available in steel designations, so strictly speaking the "S" designation only differentiates B36.19M pipe from B36.10M pipe.

piping codes & standards

The integrity of a piping system depends on the considerations and principles used in design, construction and maintenance of the system. Piping systems are made of many components as pipes, flanges, supports, gaskets, bolts, valves, strainers, flexibles and expansion joints.The components can be made in a variety of materials, in different types and sizes and may be manufactured to common national standards or according a manufacturers proprietary item. Some companies even publish their own internal piping standards based upon national and industry sector standards.

Piping codes and standards from standardization organizations as ANSI, ASME, ISO, DIN and others, are the most common used in pipes and piping systems specifications. Here are some abbreviations:

ASME ASTM ANSI AWWA API BSi DIN ISO JIS	American Society Of Mechanical Engineers American Society for Testing and Materials American National Standards Institute American Water Works Association American Petroleum Institute British standards and specifications Deutsches Institut für Normung International Organization for Standardization Japanese Industrial Standards
B31.1 B31.2 B31.3 B31.5 B31.9	Power Piping Fuel Gas Piping Chemical Plant And Petroleum Refinery Piping Refrigeration Piping Building Service Piping
PPI AWS PFI MMS	Plastic Pipe Institute American Welding Society Pipe Fabrication Institute Manufacturers Standardization Society of Valve and fitting Industry

 

Piping connection types:

BUTT WELDED

SOCKET WELDED

SCREWED

The buttwelding ends are prepared by beveling each end of the valve to match a similar bevel on the pipe. The two ends are then butted to the pipeline and joined with a full penetration weld.This type of joint is used on all valve styles and the end preparation must be different for each schedule of pipe. These are generally furnished for control valves in sizes 2-1/2-inch and larger.

Socket Weld Connection

The socket welding ends are prepared by boring in each end of the valve a socket with an inside diameter slightly larger than the pipe outside diameter. The pipe slips into the socket where it butts against a shoulder and then joins to the valve with a fillet weld.

Socket welding ends in a given size are dimensionally the same regardless of pipe schedule. They are usually furnished in sizes through 2-inch.

 

 

 A Welding Procedure Specification (WPS) is a formal written document describing welding procedures, which provides direction to the welder or welding operators for making sound and quality production welds as per the code requirements . The purpose of the document is to guide welders to the accepted procedures so that repeatable and trusted welding techniques are used. A WPS is developed for each material alloy and for each welding type used. Specific codes and/or engineering societies are often the driving force behind the development of a company's WPS. A WPS is supported by a Procedure Qualification Record (PQR or WPQR). A PQR is a record of a test weld performed and tested (more rigorously) to ensure that the procedure will produce a good weld. Individual welders are certified with a qualification test documented in a Welder Qualification Test Record (WQTR) that shows they have the understanding and demonstrated ability to work within the specified WPS.