Installing Impulse Piping for Pressure Transmitters

What is Impulse Piping?
The impulse piping is the one which connects the process outputs to the transmitter.

It must convey the process pressure accurately. If for example, the gas collects in a liquid-filled impulse line, or the drain of a gas-filled impulse line becomes plugged, it will not convey the pressure accurately. Since this will cause errors in measurement output, selecting the proper piping method for the process fluid (gas, liquid or steam) is very important. We discuss below some of the most common routing principles for impulse piping.

Impulse Piping Connections for
Differential Pressure Transmitters

Differential Pressure Transmitters
In differential pressure transmitters, there is a chance that process fluid (liquid, gas or vapors) may accumulate inside the impulse piping that can cause inaccurate reading of pressures. There are three cases as depicted by the figure.

1. Liquid
If the process fluid is liquid, the transmitter should be placed lower than the taps.

2. Gas
If the process fluid is gas, the transmitter should be placed higher than the taps.

3. Steam
If the process fluid is steam, it has more chances to vaporize, so we should use condensate pot and the transmitter should be placed at lower level than the taps.

Impulse Piping Connections for
Absolute/Gauge Pressure Transmitters

Open Tank/Closed Tank

Process Piping Connections

Reference:

Yokogawa EJX Series Differential Pressure Transmitters Installation Manual

IM 01C25A01-01E

1st Edition

What is Blowcase

A blowcase is a pressure vessel that collects fluid at low pressure, then upon fillage is pressurized with high pressure gas, causing the collected fluid to dispel into a medium pressure system. The blowcase performs the same function as a pump, only it utilizes high pressure gas to displace fluid. They have been used many years for transferring fluid, and were typically installed where there was an ample supply of high pressure gas, such as waterfloods and large oil fields where gas lift was employed. In this latter case, satellite separation and testing facilities were often needed to perform well tests and separate fluid for transfer into separate collection systems. To reduce the backpressure on producing wells, and allow use of low pressure separation equipment, blowcases transferred fluid from low pressure separators into the higher pressure collection lines – without pumps or tanks. Another typical installation is compression of “wet” gathering lines, where fluid must be removed prior to compressing the gas, and re-introduced downstream of the compressor. Perhaps the most common applications at present are the use of five gallon vessels to collect compressor skid drain liquids, and glycol reconcentrator condensation for transfer to larger storage tanks.

The prevention of atmospheric storage tank vapor losses, and prevention of oxygen entry has resulted in additional blowcase applications in “wet” gathering/compression facilities. Instead of directing all separator liquids to atmospheric tanks, installing gas blankets and vapor recovery compressors, and finally pumps to remove the fluid, some operators elect to install blowcases. Tanks are sometimes installed as emergency backup. Economics favor the blowcase, since it usually costs far less than gas blankets, vapor recovery, pumps, and chemicals.

Taken from : One Petro

Startup Problem

Courtesy: controlglobal.com

Stuxnet: Siemens releases a removal patch

Stuxnet is said to be a special kind of virus that is directed towards Siemens control systems. It can entered a system via a USB and other mobile means and looks for the Siemens PLC connection with it. Once found, it modifies its logic to an uncertain extent that may cause devices to shutdown, or even may explode. Some websites claim this virus to be a myth but when i saw Siemens releasing a security/cleaning patch, i can say that there is something really like Stuxnet that exists today. You can read and download it here:

Siemens Security/Cleaning Patch for Stuxnet

If this really is a truth then this can bring a unique and a new threat revolution in the world of Industrial control systems. These type of viruses can really proved to be a disaster for the plants as modifications in the logics can cause shutdowns, inappropriate control or may be serious accidents. It is important to install proper firewalls and  antivirus programs in order to keep our control servers cleaned from such attacks.

End of Line Resistance

Older systems used to have a redundancy built into the wiring, 2 wires would go in and out of all of the devices and then return to the panel, this used to be called class "A" wiring (NFPA now refers to this as Style D or Style Z wiring).

Most current alarm systems use a resistor after or at the last device on a 2-wire circuit. (Class "B" wiring, or Style B and Style Y wiring under the newer NFPA 72.) The control panel or control device on an addressable system is constantly looking for that resistance as a "normal" condition. Conventional Smoke detectors, for example, will lower this resistance when activated to a point below the alarm threshold and place the panel in alarm. Pull stations and most heat detectors will short the circuit (in the U.S.) also creating an alarm (a short is basically zero resistance, although the wire itself offers a small amount).

If a wire connection is loose or is cut, the panel or device stops "seeing" the resistor and enters a fault or trouble condition. Normally, this is then looked into by a maintenance man or authorized service company. 

Handshaking in Serial Communication

This EIA-232 communication method allows for a simple connection of three lines -- Tx, Rx, and ground. However, for the data to be transmitted, both sides must be clocking the data at the same baud rate. Although this method is sufficient for most applications, it is limited in responding to problems such as overloaded receivers. This is where serial handshaking can help. Three of the most popular forms of handshaking with EIA-232 are software handshaking, hardware handshaking, and Xmodem.

Software Handshaking
This method uses data bytes as control characters similar to the way GPIB uses command strings. It also incorporates the simple three-line set of Tx, Rx, and ground because the control characters are sent over the transmission line like regular data. With the SetXMode function, you can enable or disable the use of two control characters, XON and XOFF. The data receiver sends these characters to pause the transmitter during communication.

The biggest drawback to this method is also the most important fact to keep in mind -- decimal 17 and 19 are no longer available for data values. This typically does not matter in ASCII transmissions because these values are noncharacter values; however, if you transmit the data via binary, it is very likely you could transmit these values as data and the transmission would fail.

Hardware Handshaking
This method uses actual hardware lines. Like the Tx and Rx lines, the RTS/CTS and DTR/DSR lines work together. One is the output and the other is the input.

The first set of lines are RTS (Request to Send) and CTS (Clear to Send). When a receiver is ready for data, it asserts the RTS line, indicating it is ready to receive data. This is read by the sender at the CTS input, indicating it is clear to send the data.

The next set of lines are DTR (Data Terminal Ready) and DSR (Data Set Ready). Engineers use these lines mainly for modem communication because they allow the serial port and the modem to communicate their status. For example, when the modem is ready for the PC to send data, it will assert the DTR line, indicating that a connection has been made across the phone line. This is read in through the DSR line, and the PC can begin to send data. The general rule of thumb is to use the DTR/DSR lines to indicate the system is ready for communication and the RTS/CTS lines for individual frames of data.


Reference: National Instruments

PID Bumpless Transfer

Bumpless Transfer in PIDs means that switiching between auto and manual modes of PID without upsetting the output value. In PLC it is done by loading the PID output register value in a spare register while in Auto mode. Once switched to Manual, the value in register is loaded in the PID output register. This way the output doesnt get disturbed.

In DCS we have to simply allow the optin of "SP Tracking PV".

HART Communicator 375 not accepting temperature transmitter

One day we experienced a strange problem. Our temperature transmitter was not giving proper values. We connected it with our HART Communicator 375 but it too was not behaving normally. What it was doing that it was getting the device disconnected again and again. The device was actually a temperature transmitter connected with an RTD.

We brought HART Communicator 275 in and test the transmitter with it. It picked the device correctly. When we explored the settings, we came to know that the transmitter was configured as of THERMOCOUPLE type. We changed the configuration to RTD type we were using and the transmitter then started behaving properly. We then tested it with HART Communicator 375 and it picked that device correctly now.

We feel that it might be some issues with the backward compatibility of 375. What do you say?

Burner Lockout Problem in Heating Step: Water Bath Heater

What is Water Bath Heater

The main application for Indirect Fired Water Bath Heaters is to heat high pressure gas prior to pressure reduction, this prevents hydrate formation that can occur because of the temperature drop due to the Joule Thomson effect. The natural gas can also be post heated to suit the operation of gas turbines.

A typical water bath heater consists of an insulated shell, removable process coil, removable fire tube, stack burner, gas train and control system. (Courtesy GASCO)

We were assigned a job regarding a water bath heater of a natural gas processing plant. It was tripping due to burner lockout problem during heating step.

The problem was found actually in the AFR (Air to Fuel Ratio) controller. It was a manual controller. It was found that its seed was actually blocked with wax. We took that seed out, cleaned it and reinstalled it and it worked perfectly since then.


Remember that we were using UV based flame detector which was tripping the heater again and again because of a very small flame. The flame was smaller because its path was not totally cleared (due to wax in the seed of AFR controller).

Control Valve Parts

Developed by: RS

Industrial Instrumentation and Control

Welcome Fellows!

In today's modern world, the top target of every plant management or plant ownership is to achieve the safest and maximum production out of their equipment, machinery and devices. In order to reach that target, one of the major steps it follows is to install good instrumentation in the plant. When we walk through a modern day plant, we find many transmitters, gauges, trolls, control valves, motors, solenoid valves etc. This is all nothing but Instrumentation.

But the fact is, that instrumentation can't do anything alone. It needs some brain that can constantly monitor the readings provided by it and takes actions accordingly to operate the final control elements like valves, motors etc. At the same time, these actions must ensure to operate these devices at optimal parameters as to ensure their health and long-life. This is where the world of control comes in.

Control System enables the instrumentation to operate in a way to give you the safest and optimum production. Nowadays, it is fully automatic. Once configured, it brings your whole plant over the screens of control room.

This blog has been developed in order to discuss about these two greatest technologies of industrial plants. We will be sharing our experiences, our ideas and our observations. We invite you to be the part of it to contribute it to be the world's best resource of learning and sharing the experiences of instrumentation and control.

Lets control it now!

Regards

SB