Learn How to Read P&ID Drawings – A Complete Guide

Reading P&ID is a difficult task for those who start their career in Oil &Gas and similar Chemical Process Industries. Here, I have tried to explain P&ID and PEFS an easy way.

Piping and instrumentation diagram is also known as Process engineering flow scheme which is PEFS.

You will learn how to read P&ID and PEFS with the help of the actual plant drawing. P&ID is more complex than of PFD and includes lots of details.

What is P&ID?

P&ID is a graphical representation of the actual process plant using various symbols that represent actual equipment. As said earlier it is complex than PFD. A single PFD can have multiple P&ID. This means if some system is shown on single PFD, to show the same system on P&ID, it may require multiple P&ID sheets.

It is one of the main deliverables of FEED. That is front-end engineering and design. It is used during the entire life-cycle of the plant. That means during pre-EPC, EPC and operation.

Use of P&ID/PEFS during Pre- EPC Phase

P&ID is used to derive Project capital cost estimate. It is also used to develop EPC contract specification. Plot plant is developed considering various inputs from P&ID and physical site location.

Use of P&ID/PEFS during the EPC phase

During the EPC phase, P&ID is used to develop the individual unit layout. It used to identify hazardous areas classification, preparing data sheets of equipment, valves, and instrument.

P&ID are used to develop the piping layout and preparing bulk material take-off for piping, electrical, instrumentation and civil. It is key documents for various review such as HAZOP, SIL and operability review.

During Operation, you have to maintain P&ID in such a condition that it will show actual plant conditions at any time. It should be updated when any physical change is made so that the unit will remain compliant with codes, standards, and specification, and can be operated safely under the defined process conditions.

P&IDs are used to train operators and engineers before they start work in the plant.

What information does P&ID provide?

  • All the equipment, including installed spares, and associated piping including drain and vent line.
  • Insulation or jacketing requirements.
  • Instrumentation
  • Heat tracing and insulation detail
  • Information about utilities
  • Piping components including their size, class and tag Number
  • Information required for design, construction, and operation such as
  • A slope of the line
  • Minimum and maximum distance from the equipment or instruments
  • Minimum straight lengths after instruments

What is not included in a P&ID?

  • Process conditions and physical data
  • Operating conditions
  • Stream flow details
  • Equipment locations
  • Pipe routing, length, and fittings
  • Support and structural details are also not included in p&id

Ok, now you know what P&ID is and types of information you’re going to get from the drawing. Let’s confirm this with the help of actual P&ID.

This is a PFD of the flushing oil system shows the entire system of pump seal flushing oil. This is the simplest system with just one cone roof tank and two centrifugal pumps. The pump used in a heavier product such as crude, fuel oil required flushing oil to keep the pump seal clean. Here Ultra low sulfur diesel is used as flushing oil.

You can see here that ULSD is coming from diesel rundown line to the tank and with the help of pump it is supplied to the various pump of ISBL and OSBL units’.

Hope the function of the system is clear to you. Now let’s move to P&ID.

There are two P&IDs for OSBL part of this system and May more for ISBL parts. I will explain you the OSBL part of the system.

How to Read P&ID Symbols 

How to Read PFD

If possible, get a print of this P&ID in A3 and follow the video. You can download this P&ID the link is given at the end of the article.

It is always a good practice to start reading P&ID from the main incoming lines and follow the fluid path.

Line Tracing – Mani Incoming Line

Here ATF is coming from CDU; you can see that. Remember the black head on the arrow? You have learned this in how to read PFD video. It means Diesel is coming from the different unit. Here you can see the line number. It is 150 mm diameter line as per DN standard which is equivalent to 6” NPS.

The different company follows different terminology for the line number. But it contains same information such as line size, unit number, commodity code that identify fluid inside the line, circuit number, line sequence number, piping class that gives all detail about piping components and their materials, insulation, and coating requirement.

Here N means there is no insulation. If there is H than hot insulation, C is for cold insulation, A for acoustic reduction, P for personal protection, F for fireproofing. It also gives information about whether the line is steam trace or electric trace. Normally ET is used for electric tracing and ST is used for steam tracing.

Let’s move ahead, here you can see that the diesel line is divided into two strim. We follow this strim. 6” line is getting reduced to 4”. Don’t get confused with reducer symbol; it is just an indication of line size change. In piping, it can be a reducing tee as it is 6” to 4” concentric reduction.

Now, this triangle with line indicates a line break. A line break is the demarcation of the line number change. So, whenever this symbol is used it indicates that from that point onward line number is different.

This is the bypass loop for the flow transmitter. You can see that the venturi-type flow transmitter provided in between the two gate valves. Why did I say it is a venture type flow transmitter? Letters VM indicates the type of flow transmitter. It gives flow indication on the control plane. You can see the connection shown between FT and FI.

From the piping point of view, you can see that there is a 25 mm drain valve is given in between the two gate valves. The letter D indicates that it is a drain valve and if it is V than it is a vent valve. Off course if it is a vent, it shows on the topside, not on the bottom. The bypass valve is also a gate valve which will remain closed during normal operation. You can see the letters NC which indicates the same.

MOV Symbols

Reading P&ID is nothing but reading P&ID symbols. So, if you are aware of symbols, you can easily understand the P&ID.  Here you can see the motor-operated butterfly valve. Lots of instruments are shown here. But it is not that complicated. If you are aware of MOV, you know that it can be operated locally or from the control penal. You can read AT VALVE label that means this instrument function is available on the valve itself.

HS indicates hand switch, with these switches you can put MOV on manual control or remote control that means on panel control. You can also start or stop the valve from the field. HS-O means open, and HS-C means close. The data link indicates this valve is connected to the control panel.

The last two instrument bubbles show the potions of the valve. From field, you can set and see the % of the valve opening.

Next is a drain valve that is located at the lowest point. This is the spectacle blind with a normally closed configuration. If the dark ring is towards the valve, as in this case, it indicates that solid ring covers and isolate the joint during normal operation.

If the only circle is towards the valve, as shown here, that means the hollow ring is there, and the line is live during the normal operation.

Below the drain valve, the funnel is shown. So, when you drain ATF it will go to COC system. COC means continues oil contaminated that used to drain hydrocarbon. The second type of drain is AOC that means accidentally oil contaminated system.

After a dike wall, there is a pneumatically controlled globe valve is there. You can see the pneumatic line symbol. Like MOV, the pneumatic valve also has various switch to operate the valve locally and from the control panel. You can also see the three-way valve connected to the pneumatic line to operate the actuator.

You can refer to the abbreviation table to understand the meaning of all these instrument bubbles. I have attached this table with a free download. The link is given in the description.

P&ID Abbreviation Table

You can see that there is a relief valve in bypass. It used to protect the actuator from the surge.

Main Equipment – Tank

Next is a tank. First, I will explain mechanical parts and then instrumentation. As you can see, this is a fixed roof tank. On the top of the P&ID, you can see the detail of each of the equipment shown in the drawing. Let see the detail of this tank.

The tank is 17.5 meter in height with 8.25-meter diameter. You can also see the operating pressure and temperature of the tank. The material of construction is carbon steel, and there is no insulation. Now let go back to the tank.

N1 to N17 are nozzle number. P&ID don’t show the exact location of the nozzle, but it shows the size of the nozzle. Manholes are shown as M1 to M3. The broken line shows the internal piping. There is a vortex breaker with the N8 nozzle connected to pump suction line with a Normally Closed gate valve. This is because the main suction line is N2. N8 will use only when you want to drain the tank completely.

There are two more nozzle N9 and N10 that used to drain the tank to COC.  This is open type liquid seals that prevent air ingress into the tank.

Now let’s check the instrumentation. For safe operation of any equipment, you have to monitor pressure, temperature, and level. Here you can see the Pressure Transmitter near the tank bottom plate. On the top of the roof, you can see the radar type level indicator and transmitter.

The temperature gauge is shown as TE that is temperature elements. This arrangement shows multiple thermocouples installed at a different height of the tank. This will ensure that you will get an average temperature of the tank as the liquid has a different temperature at a different level.

What is LLL, HLL, and HHLL in Tanks and Equipment?  

LZT is level safety transmitter. Here you can see the value for LLL, HLL, and HHLL. LLL means low liquid level, HLL means high liquid level and HHLL means high high liquid level. LLL protects your pump and when tank level reached to LLL, it gave the alarm in control penal and based on the logic configuration it may trigger to switch off command to the pump.

HLL and HHLL will use to protect the tank from over filling. When the liquid level reached to the HLL it gives the alarm and when it reached to HHLL it will trigger the safety logic and stop the fluid supply to the tank.

There is a breather valve on the tank. It will protect the tank from the overpressure and vacuum.

Now, these all instrument bubbles are sending the information to the local and main control panel for the pressure, level and temperature instrument that I have explained to you. You may have noticed that there is a tank gauging system which received the all input signal from the instruments.

Tank gauging system is used to calculate the quantity of the liquid stored in the tank at any given time. Based on the diameter, level, and temperature it will calculate the quantity of the liquid stored in the tank. QI is a quantity indicator.

In the last part of this video, let check what is going out of the tank. AFT from the tank is supplied to the pump with the help of 150 mm pipeline. You can see that inside dike there is a manual gate valve with a bypass arrangement. There is a safety relief valve on the bypass.

Outside the dike, you can see the motor-operated butterfly valve. This MOV has similar switches that I have explained to you earlier to operate the valve locally and from the control panel.

If you know the instrument legends and symbol, you can read and understand any P&ID.

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