How to Read the Hookup Drawing of a DP Type Level Transmitter?

One of the most important skills in the life of an instrumentation engineer and technician is the ability to read and interpret DP type level transmitter hookup drawings. These drawings provide detailed instructions for installing and configuring DP level transmitters, ensuring accurate measurement and integration within a system.

 The following is a step-by-step way of reading a hookup drawing of a DP type level transmitter, to be followed by reading an example drawing in more detail.

Step-by-Step Procedure for Interpreting the Hookup Drawing of a DP Type Level Transmitter:

Step 1: Title Block and General Information

Title:

• The title block, usually located at the bottom right corner, provides the type of equipment or instrumentation. In this case, it indicates that the drawing is for a “DP Type Level Transmitter.”
• This helps in quickly identifying the drawing’s purpose.

Drawing Number:

• Found within the title block, the drawing number (e.g., “DWG.NO. STD-4-DPLT-2000”) is essential for reference and document control.
• Each drawing is uniquely identified within a set of documents.

Revision and Sheet Number:

• These details are also in the title block.
• The Revision Number indicates the version of the drawing (e.g., Rev 1, Rev 2), while the Sheet Number identifies its place within a set (e.g., “Sheet 2 of 4”).

Company Information:

• The title block also includes the organization responsible for the drawing. This is useful for accountability and contact information.

Step 2: Referring to the Notes Section

• The notes provide specific instructions and important details about the installation.
• Example: A note might specify that the DP transmitter should be installed at a specific height or orientation to ensure proper operation.
• Another note might define the type of tubing or wiring to be used, ensuring compatibility with the process conditions.

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Step 3: Identify the Process Flow Direction

• Look for arrows on the drawing that indicate the direction of the process flow, which is crucial for correctly orienting the DP transmitter and its associated components.
• Ensuring that the high-pressure (HP) and low-pressure (LP) sides are connected properly according to the flow direction is essential for accurate level measurement.

Step 4: Main Components and Symbols

Component Symbols:

• Go through the drawing to identify various symbols representing components such as the DP transmitter, isolation valves, tubing, and process connections.
• The symbols are usually explained in a legend or within the drawing itself.
• For example, a symbol might represent a three-way manifold valve, which is commonly used with DP transmitters.

Interconnection Lines:

• Solid lines typically represent the piping or tubing connecting the DP transmitter to the process.
• Dashed lines may indicate signal or electrical connections, such as the wiring between the transmitter and the control system.

Step 5: Referring to the Bill of Materials (BOM)

• The BOM, often found at the bottom or side of the drawing, provides a detailed list of all components required for the installation.
• This includes item numbers, descriptions, sizes, quantities, and material specifications.
• Cross-reference the BOM with the drawing to confirm the placement and connection of each component.

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Step 6: Connection Details

• Examine the lines in the drawing to understand how each component is connected.
• Solid lines typically represent the physical piping or tubing, while dashed lines may indicate signal or pneumatic lines.
• Ensure that connections are made according to the flow direction and that high-pressure and low-pressure connections are correctly aligned.
• Signal cables, often shown as dashed lines, connect the DP transmitter to the control system, allowing for data transmission and monitoring.

Step 7: Scope of Work

• The drawing is often divided into sections such as “Process Scope” and “Instrument Scope,” indicating the boundaries of work for different teams.
• This helps in understanding the responsibilities of each team and ensures that the installation is performed correctly.

Step 8: Dimensional Requirements

• Pay attention to any dimensional notes, such as the required distances between the DP transmitter and process taps.
• Proper spacing is crucial for accurate measurement and to avoid errors due to turbulent flow or incorrect pressure readings.

Step 9: Installation Specifications

• Review any specific installation instructions, such as the positioning of the transmitter relative to the process taps, to ensure that the setup will function correctly under operating conditions.

Step 10: Component Details

• The drawing may include callouts that provide additional details about each component, such as material specifications, pressure ratings, or connection types.
• These details are essential for selecting the correct components and ensuring compatibility with the process conditions.

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Detailed Analysis of the Sample Hookup Drawing

Title Block

• Company: xxxxxxxxxxxxxxxx
• Title: Level Transmitter – DP Type.
• Drawing Number: STD-4-IHU-1000.
• Sheet Number: 34.

Main Components:

1. Equipment (01): This represents the process vessel where the level measurement is to be taken.
2. Nozzle (02, 03): These are the connections on the vessel through which the process fluid reaches the transmitter. Typically, there are two nozzles: one for the high-pressure (HP) side and one for the low-pressure (LP) side.
3. Isolation Valve (03): A ball type isolation valve made of Carbon Steel/Stainless Steel, sized 1/2″ NPT (F) is used to isolate the process from the measurement device during maintenance or calibration.
4. Seamless Tube (07): A 1/2″ O.D. SS-304 seamless tube that connects the isolation valve to the DP level transmitter.
5. Condensate Pot (05): This is used in wet leg measurement systems, ensuring that there is always a column of liquid in the impulse line to prevent vapor from entering the transmitter.
6. 3-Way Manifold (08): A T-type 3-way manifold made of Stainless Steel, 1/2″ NPT (F), is used to zero the transmitter and for venting and calibration.
7. Differential Pressure (DP) Level Transmitter (09): This is the primary instrument that measures the level by comparing the pressure difference between the high and low-pressure sides.
8. Cable Gland (10): A 1/2″ NPT (M) cable gland is used to secure the signal cable entry to the transmitter housing.
9. Signal Cable (11): This connects the transmitter to the control system, allowing for remote monitoring and control.
10. Stanchion (12): A support structure that holds the transmitter and other associated components at the correct elevation.

Flow and Connections:

High-Pressure (HP) Side:

• The high-pressure process fluid from the vessel enters through the nozzle (02), passes through the ball-type isolation valve (03), and flows through the seamless tube (07).
• The fluid is then routed through the condensate pot (05) and the 3-way manifold (08), and finally, it reaches the high-pressure port (HP) of the DP level transmitter (09).

Low-Pressure (LP) Side:

• Similarly, the low-pressure process fluid enters through nozzle (03) on the vessel, passes through the ball-type isolation valve (03), and flows through the seamless tube (07).
• It is routed through the condensate pot (06) and the 3-way manifold (08), and then it reaches the low-pressure port (LP) of the DP level transmitter (09).

Signal Cable (11):

• The transmitter output signal is connected via the signal cable (11) to the control system for remote monitoring and control.

Notes:

• Transmitter Elevation: The transmitter must be installed below the lower measuring point to ensure accurate level measurement.
• Accessibility: The transmitter should be installed in an accessible location for ease of maintenance.

Bill of Materials (BOM) with Location Number

The BOM provides a detailed list of all components required for the installation, ensuring nothing is overlooked:

General Recommendations for Reading Hook-up Drawings of a DP Type Level Transmitter

Know the Application Background

• Make familiar yourself with the application of the transmitter, such as whether it is used for liquid level, interface level, or other usage.
• Understand pressure, temperature, and fluid properties to ensure the correct materials and components are used.

Familiar with Standard Symbols

• Become familiar with all standard symbols of equipment used in your industry, such as ISA or IEC.
• Always refer to the legend and abbreviation list for correct deciphering of symbols and notations.

Cross-reference with P&ID

• Compare the hookup drawing with the P&ID to understand the DP transmitter in the system.

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Instrumentation Loops

• Identify and trace the loop number so the signal from the transmitter can be followed to the control system.
• Identify the signal type (e.g., 4-20mA, HART), including associated barriers or transmitters.

Installation Environment

• Survey corrosion, temperature, and vibration in the environment to select materials and protective enclosures.
• Ensure adequate space is available for maintenance and calibration activities.

Electrical and Signal Connections

• Check grounding and shielding requirements to prevent interference from electrical noise.
• Check power supply specifications to ensure compatibility with the control system.

Process Connection Details

• Check the routing and sloping of impulse lines to prevent pockets of air or liquid.
• Check the locations of tap points to avoid turbulence that may affect accuracy.

Materials of Construction

• Ensure that all materials are suitable for service conditions and process fluid characteristics, and meet industry standards.

Calibration and Commissioning

• Identify calibration ports and know the calibration procedure for the DP transmitter.
• Identify the procedure to make zero and span adjustments.

Documentation and Record Keeping:

• Mark up any changes made during the installation and update the drawings.
• Ensure that all devices are correctly tagged for reference.

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