LMTD, Log mean temperature difference is the temperature driving force for heat transfer in flow systems. When you are calculating heat transfer by overall heat transfer coefficient (U), you should not simply multiply temperature difference of hot stream or cold stream. Multiplying this temperature difference doesn't account the overall driving force.
Tuesday, May 12, 2015
Pressure drop due to valves and fittingsIn the earlier post, tutorial goes around pressure drop in pipe lines. Pressure drop arises due to skin friction and form friction. Non separated boundary layers cause skin friction whereas separated boundary layers cause form friction. In plain pipe pressure drop is due to skin friction. But in a pipe line with valves and fittings pressure drop is mainly due to form friction. the pipe lines may be connected to equipment and again pressure drop will be there in that equipment. This tutorial is restricted to pressure drop in valves and fittings only.
Wednesday, March 11, 2015
Wednesday, February 25, 2015
Earlier two parts of designing a P&ID were from process perspective of a P&ID. Now in this present part we will deal with the instrument aspects of a P&ID. Some of the most common instruments which we are using in process industry are transmitters, gauges, indicators, control valves, on off valves etc. The parameters which we are interested may be flow, temperature, pressure, concentration etc. Before going to discuss further about instrumentation part in P&ID it is better to get familiarized with the instrumentation terms. Let’s get started
Tuesday, February 10, 2015
Friday, February 6, 2015
Designing a P&ID Part-1
In this part we will see the legend and symbols using in P&ID now days. Please note that the legend and symbols may vary from one process designer to another and the reader should understand that. Following sections will give a brief idea about P&ID representation.
Pipe line number
X” – AB – CDEF – GHI – J
X = Pipe line size.
AB = Service Code (Arbitrary for Chlorine I can take as CL).
CD = Last two digits of drawing number.
EF = Line sequence number.
GHI = Line material of construction.
J = Insulation thickness.
6” – CL – 0102 – CS – 40H
The above line is 6 inch pipe in chlorine service present in a drawing whose number ends with 01 and it is second such line in the drawing. Also the material of construction is Carbon Steel with 40mm thick hot insulation.
Service code, pipe material, insulation notation varies from one company to another. Some companies also use color coding to easily differentiate the lines in colored P&ID.
In order to easily review a section in P&ID it is always better to allot specific number for that particular unit. Consider the following example.
31: Raw material processing section.
32: Reacting section.
33: Purification section.
34: Storage/Packing/Drumming section.
So if I say, I am using a roll crusher in 31 section, I am actually referring roll crusher present in raw material processing section.
Equipment tag format
In a process industry we use lot of process equipments (pumps, vessels, exchangers, reactors etc,). Calling a pump with its true name like “Propylene glycol distillation column bottom pump” may create confusion. Also it is cumbersome enough to mention with its original name all times. So we use a notation which conveys the equipment name tidy. Generally the equipment tag name will be of the form
AN – EQ – SN
AN: Area Number
EQ: Equipment type code
SN: Sequence Number
03 C 3204
The above tag name corresponds to a fourth (04) column (C) in reacting section (32), whose area number is three (03).
Again some companies follow three digit tag number and some four digits. It is purely based on the company discretion. For equipments we can use the following notation.
Distillation columns, absorption columns, scrubber, packed columns
Thursday, February 5, 2015
What is P&ID?
A P&ID or a piping and instrumentation diagram is nothing but a diagram which depicts all the pipe lines and instruments connecting various process equipment and vessels. Here in these diagrams we can see the pipe line size, service, material of construction and the line number from piping point of view. Also the number of valves and their type using for a particular project are finalized based on the P&ID. For different types of process equipment and fittings we use various symbols to differentiate their purpose. Certain operating conditions like temperature and pressure are also represented near critical areas like pressure safety valves. The above discussion revolves around the process engineering.
Coming to the instrumentation part, just think about how you will measure the flow, temperature, pressure etc., in a pipe line which is flowing at higher temperature and pressure. You can’t take a chance to measure all the parameters by your hand 24*7 and report to DCS (Distributed control system) that’s funny right. That is where you need the help of instruments like transmitters, indicators, recorders, alarms etc., if you want to know the process parameters in various places of the plant you need to mount these instruments at the respective areas. Some of them may be locally displaying instruments and some may send the parameter details to DCS. You need to show all the instruments in the P&ID with various symbols and notations for local, DCS instruments.
P&ID diagrams also embody the control mechanisms involved in the process. For example if a low level in a vessel should close the discharge line from that vessel, the level transmitter in that vessel should send a signal to the control valve in the vessel discharge line. This can be easily represented by the following drawing.
Another aspect of P&ID is the interlock representation. Interlocks are very critical and vital for the safe operation of the process plant. The open loops and closed loops look after the normal operation of the plant whereas the sequence and logic look after the emergency shutdown functions. These interlocks are also represented in the P&ID. For tutorials on P&ID development keep watching the site www.chemineering.blogspot.com.