Pressure and Temperature for Operating, Design and Test Conditions of Piping.

Process parameters are one of the very first essential inputs with which work of piping engineer begins. Process parameters basically mean pressure, temperature, flow rate, enthalpy (Heat content of fluid), viscosity, density of the fluid flowing inside the pipe. All these and other more parameters are determined by process engineers to design and size various piping systems and equipment. For piping engineer, these parameters are considered very important to prepare various piping related calculations, documents and deliverables. They are needed to calculate size of the pipe, Wall thickness of pipe, pressure drop calculation, to select appropriate piping material and its pressure temperature rating, pressure test procedures. Process parameters are essential to raise RFQs for various piping componenets and equipment. Vendors can not design or recommend appropriate product without this information. They are most essential input to carry out pipe stress analysis calculations. Process parameters need to be finalized at the early stages of project, because any change in those parameters could have cascading effect on changes in subsequent design activities, project schedule and cost. And if they get changed at the procurement stage of the project it could result in “change order” for goods that have already been ordered. This could lead to lot of rework.

So, what are the plant operating conditions?

Any type of process plant, be it chemical, food, pharma, power, oil and gas, industrial, mining, does not run on its fixed capacity all the time. With demand and requirement of production they run on their partial capacity. The demand could vary according to season, service requirement, market demand, and even according to time of the day.

Various partial capacities on which these plants operate are called its operating conditions. e.g. 30% condition, 50% condition, 60% condition, 90% condition, etc. When these plants operate on partial capacities, their process parameters too change according to each operating condition. so, there is unique set of parameters for each operating condition. Value of each parameter changes as operating condition changes. Higher operating condition does not always imply higher value of operating parameters and lower operating condition does not always imply lower value of operating parameters.

Maximum and Minimum Operating Conditions.

For the purpose of design, calculations and sizing of various equipment and piping systems, maximum and minimum value of all parameters across all operating condition is taken in to consideration. Set of maximum values of each parameter across all operating conditions is called Maximum Operating Parameters. And similarly set of minimum values of each parameter across all operating conditions is called Minimum Operating Parameters.

Design Conditions.

Design Conditions are determine based on the most severe operating conditions that could occur during entire range of plant operating cycle. Some safety margin or factor of safety is considered over and above value of these parameters to arrive at Design Condition Parameters. There are some criteria and practices considered while determining these parameters. Design parameters are the parameters that are used as basis for all the engineering activities related to that piping, component or equipment.

Design Pressure

Design Pressure is the maximum or minimum operating pressure that might occur in the system during entire range of operation plus some margin. This design pressure is used to do all type of calculations for pipes.

Design pressure of the piping depends on the design pressure of the equipment it is connected to. Various codes and industrial practices dictate specific criteria for each type of equipment to determine its design pressure. Design pressure of the piping connecting these equipment should not be less than their design pressure.
Design pressure of the piping should not be less than the set pressure of pressure relieving device it is connected to. Pressure relieving devices are provided on the piping where pressure is expected to rise above the design pressure of the piping during some abnormal operating scenario. When this situation occures, pressure releaveing divices pop up and releases pressure before it can react to piping design pressure. Hence, the pressure at which this divices are set to pop needs to be lower than the desing pressure of the piping. This is done for the protection of the piping.
According to one of the engineering practice design pressure of the piping is usually considered 10% higher than the maximum operating pressure of the fluid. e.g. if the maximum operating pressure of the fluid is about 15 bar, then the design pressure should at least be 15+1.5 = 16.5bar. This percentage margin could be different for different projects, clients, companies, countries, piping systems, process, type of fluides. For example, if the piping system is handling some leathal fluid, this margin could be higher. Many examples can be given for each of the scenarios mentioned above. But the point is this margins are mentioned, discussed, and approved in the design criteria documents before useing them for further engineering activities.
According to another engineering practices, when maximum operating pressure of fluid is in lower range such as 2-3 bars or muchh higher range such as 100-150 bars, instead of considereing 10%, some reasonable margin is added to maximum operating pressure to arrive at design pressure. For example, if your piping operating pressure is 150 bar, then 165 bars design pressure would be unnecessaryly too high. If piping system does not have any possibility to face pressure of 165 bars in any of the worst scenarios, then designing piping for that pressure would be an over design and waste of money. Hence, some reasonable margin is considered, say, 5 bars and design pressure is determined to be 155 bars. This is done to prevent over design of piping and save cost. And as mentioned before, this margin is discussed and agreed during preparation of design criteria documents.
There are some piping services in which maximum operating pressure is in lower range, between vaccum to 2-3 bars. If you calculate the required pipe wall thickness for this pipes based on the forumlas given in the code, you will arrive at very less pipig thickness. Sometimes less than a milimeter. Such very thin walled pipes poses fabrication, maintanence and handling related chalanges. They are difficult to fabricate. Threaded connections are difficult because they do not have enough thickness to machine threads at their ends. And for the same reason, welding connections on such pipes is also a challange. Such pipes has weak structural strength. They may break or get distort easily if came under some external loads such as man movement or handling. Hence, to avoide all these complications, some minimum design pressure is considered for such piping serices. No piping in the plant has design pressure less than that. Hence, as per general engineering practice, minimum 3.5 bars design pressure is considered for such piping even when the max operating pressure is much less than that. Even if it is vaccum or same as atmospheric pressure.
Equipment or piping that is expected to operate below atmospheric pressures should be designed for full vacuum conditions unless vacuum breaker devices are provided on them. Vaccum becomes problem for very large size pipings or pressure vessals. Very large size piping such as cooling water lines, made from GRP FRP material, does not have the sufficient strength for the vaccum conditions. Such piping neeed to be provided air release cum vaccum breaker valves. Some equipments such as steam turbine condensers operate at pressure less than atmospheric pressure. Piping where such conditions occures has to be deisgned for vaccume (external pressure). For designing piping under external pressure, Piping code B31.3 gives reference to BPVC Sec.VIII Div1 UG-28 to 30.
Design pressure for pump suction piping is determined based on the pressure of the vessal from which suction is taken and static head difference between high level of the vessal and pump suction nozzle. As mentioned above some margin is added in the pressure arrived at with summation. Any pressure drop that may happen along the way in piping or in suction strainer is ignored while determining design pressure for pump suction piping.
Design pressure for discharge piping for pumps and compressors has to be determined based on the maximum pressure these equipment can generate. The maximum pressur a pump can generate is called shut off pressure. This basically means the pressure, the pipe would experience if you completely bloc the flow of fluid while pump is running. This shut off head is provided by the pump manufacturer. If it is not available, then for estimation purposes, shutoff head is considered 1.2 or 1.3 times operating pressure of the pump discharge. Some margin is added to that pressure to arrive at design pressure. If the piping is not designed for pump shut off pressure, then some pressure releaving valve need to be considered in the discharge pipe.

Design Temperature

Design temperature is the maximum or minimum temperature of the fluid that might occur throughout entire range of operating conditions plus some margin. For all piping related calculations, the design temperature is taken as input. It is most important parameter to determine pressure temperature rating of flanges (PT rating) and to determine allowable stresses for each type of piping material based on guidelines given in piping codes.

Design Temperature is usually determined by adding 5 to 10 Deg. C margin to maximum operating temperature.
How much should be minimum design temperature? well, that depends on the which type of services we are talking about and at which climatic and geographic location the project is located. For services that normally operate above ambient temperatures and are located close tropical regions, where ambient temperatures most part of the year remains above 20 to 25 Deg. C, 50 or sometimes 60 Deg. C is considered minimum design temperature. Even for services like cooling water which usually operates between 30 to 40 Deg. C also have minimum design temperature of 50 Deg. C.
When projects are located at high latitudes, far in the north, where cold climates and snow falls are common during most part of the year, possibility of fluid frizzing inside the pipe should always be considered. Very low temperatures, such as -5 Deg. C or less could have impact on piping material strength and flexibility. Metal pipes would not have much problem but plastics, PVCs and PE pipes could become brittle. In such site conditions minimum -10 Deg. C or the historically least ambient temperature at the site should be considered as minimum design temperature.
You might have heard dry bulb temperature or wet bulb temperature while studying psychrometry. There is also another type of temperature called 'black bulb temperature'. This is the temperature metal surface heat up to when it is left exposed to direct sun light on very hot summer day. Metal surface can get heated up to 80 to 85 Deg. C in such climatic conditions. Pipe surface too can get to such temperatures when they are located outside under open sky and no fluid is running inside them during shutdown time. Minimum design temperature for such pipes is considered 80 to 85 Deg. C.
Scenarios where some equipment or piping could suddenly go under depressurization, could observe sudden drop in temperature. Or places downstream of pressure reducing valves, where temperature could also drop due to sudden reduction of pressure. Such effects should be taken in to consideration while determining minimum design pressure.
There are some equipment in the plant whose temperature is being controlled by some sort of cooling medium such as cooling water, or oil, or some other type of coolant. What happens if for some unforeseen reason supply of such cooling medium gets interrupted? The temperature in such scenarios could reach up to the temperature of the hot medium which is being cooled. Design temperature of such equipment and piping should be determined for such scenarios.

Testing Conditions

Pressure and temperatures at which any piping, valve or equipment is tested is called testing parameters or testing conditions. Any piping or equipment that has been designed to withstand certain pressure during its normal and maximum operating conditions, needs to be pressure tested in order to confirm its integrity. The purpose of pressure test is to ensure that there are no leakages through welds or flanged connections, and that the piping can perform reliably at its desired operating conditions.

The pressure at which the piping is tested is generally taken 1.5 times the design pressure. It could also be 1.3 times for some of the valves. Pneumatic tests are also performed at 1.3 times design pressure. This pressure at which piping is tested is called Test Pressure.
Sometimes to save time and cost two piping systems that have different design pressure are tested together at same test pressure. In such scenario design pressure of one system is higher than the another. The piping system whose pressure is lower, can also be tested at pressure higher than the test pressure. While doing so, according to code, it is ensured that the stresses generated inside pipe material, should be within 90% of its yield stress value.
Pressure test is performed at normal atmospheric temperatures. Fluid medium could be water, Oil or compressed air depending on what kind of piping services is being tested. The temperature of these mediums is atmospheric while carrying out test.
There are elaborate procedures mentioned in piping codes regarding how to carry out pressure test. When it can be exempted. What are precautions that need to be taken during pressure test. When Pneumatic pressure test is performed and when hydro test should be performed. What kind of testing medium should be used. How long should you hold the piping under test pressure. How to find and mark the leakages. All these details are not scope of this blog article. I shall cover it in another atticle.

Uses of Design, Operating and Test Parameters.

Process parameters are used at every turn of engineering activities. It is not possible to list all the instances where these parameters are used. However, some important uses mentioned below.

Maximum Operating parameters are used to determine the fluid densities at those conditions, which in turn is used to calculate the volumetric flow rate, which then used to calculate the required size of pipe.
Maximum Operating temperatures are used to calculate the insulation thickness for pipe.
Operating Parameters are used to determine high and low set points for instruments. Design parameters need to be higher than the set points for instruments.
Process parameters need to be provided in equipment and piping specifications as input to vendors.
Design pressure and temperature are used to determine pipe wall thickness, which leads to right selection of pipe schedule and rating.
Design Parameters are used to determine PT rating of flange.
Test Parameters are used in pressure testing of the piping or equipment.
Test Parameters are one of the necessary inputs to ensure safety and code compliance.