Importance of Vents and Drains in Piping System (With PDF)

Vents and Drains are required in piping systems to meet the Process, Construction, Testing, and Commissioning requirements. These are basically a small tapping connection from the main pipes having a total length limited to 300 mm. These connections are with a isolation valve or without. Even though vents and drains are small connections in piping system and very often missed or neglected, they are very important and must be provided. Installing vents and drains after pipe has been installed results in additional cost and extended schedule. In this article, we will learn about Vents and Drains in details.

What is a Vent and Drain Connection?

A vent is a piping connection taken from the top of the main pipe whereas a drain is the tapping connection taken from the bottom of the main pipe. In a piping configuration, vents are usually taken from the high point and that is why they are known as high point vents. Similarly, drains are considered in the low points and popularly known as low point drains.

Vent and Drain Connections
Fig. 1: Vent and Drain Connections

Types of Vents and Drains

Depending on the purpose of the vent or drain connection, vents and drains are classified into two groups.

  • Hydro static test vents and drains, and
  • Process vents and drains

Hydrostatic Test Vents and Drains

Hydrotest vent and drain connections are provided for using during hydrostatic leak testing. Hydrostatic vents and drains shall be provided only for the piping system subjected to hydrotest. Vent connections are located at high points and drain connections are located at low points of piping systems based upon the physical piping configuration.

The main purpose of hydrostatic vents are to expel the air present in the pipe while filling the pipe with water for hydrotesting. If the air does not get passage to be removed, they will be compressed inside the piping system, will cause air bubble, and pressure drop will occur. Also, water venting from high point vents will ensure the complete filling of water for starting the test. Once this is ensured, these vents are closed and pressurization starts.

The hydrotesting drain connections are used to drain the water from the pipe after hydrotesting is completed. During draining the vent connection is also kept open.

If the piping system is subjected only to pneumatic test (e.g. instrument air, nitrogen system etc.), then hydrotest vents and drains need not be provided. Fig. 2 below provides some typical hydrotest vent and drain assembly details:

Hydrotest Vent and Drain Assembly Examples
Fig. 2: Hydrotest Vent and Drain Assembly Examples

Hydrostatic test vent and drain features:

  • Hydrostatic test vent and drain connections are not usually shown in the P&ID drawings but shown on the detailed construction piping drawings.
  • Hydrostatic drains are provided with an isolation valve but Hydrostatic vents are normally provided without valves. However, if valves are provided they shall be shown in the P&IDs.
  • When Hydrostatic vent and drains are marked on the P&IDs, they are tagged as ‘HV’ & ‘HD’ respectively to differentiate from the process vent & drains. Access is normally not provided for hydrostatic test drain and vents.
  • Hydrostatic vents shall be provided for line sizes 1½” and above Since line sizes 1” and below are considered to be self-venting. However, Hydrostatic drains shall be provided for all line sizes (including small bore piping).
  • The hydrostatic vent & drain sizes and arrangement shall be as indicated in the piping standard drawing or stated in the project specifications.
  • Hydrostatic vents not provided with valves, but with threaded plugs or caps shall be seal welded after testing.

Process Vents and Drains

Process vent and drain connections are provided in piping systems for use during operation (start-up, shut down) or maintenance. These are provided for specific process requirements. These drain and vent connections are shown on the P&IDs and related detailed piping drawings. Process vents and drains are always provided with valves and are located such that they are easily accessible by operations or maintenance personnel. Once the venting or draining operation is complete, these valves are closed to continue the normal operation.

Process vents and drains are closed with suitable closure devices like plug, cap, or blinds or sometimes are connected to the vent/drain systems. On the pump discharge piping a drain connection shall be provided between the check valve and block valve to permit draining of the line downstream of the check valve. This drain connection shall be indicated in the P&ID. Fig. 3 below shows some typical process vent and drain assembly details.

Process Vent and Drain Assembly examples
Fig. 3: Process Vent and Drain Assembly examples

Characteristics of Vents and Drains

Hydrotest drain shall not be provided if a line already has a process drain at the lowest point of the piping system. The process drain itself shall act as hydrotest drain in such cases.

Hydrotest vent shall not be provided if a line already has an instrument connection (e.g. pressure/ temperature instrument) at high point of the piping system. This connection itself shall act as the hydrotest vent.

Adequate gussets/ bracings shall be provided to support the vents and drains in case of lines subjected to vibrations.

Vents shall be avoided in control valve assemblies at elevated locations by using flat side top reducers. The same is applicable for outlet of relief valve lines with flat side top reducers. As during testing, the hydro tester shall provide a temporary spool with a vent connection in lieu of the control valve.

Depending on process requirements, hydrotest drains may have to be connected to the drain system.
If there are service specific drain systems like hot oil, hot water, etc., care shall be taken to connect the drains to appropriate drain system.

Separate vents and drains shall be provided on the lines in a system if piping specification or pressure class changes. With pressure break or class break, separate hydrotest packs are prepared and hence separate vent and drain for hydrotest must be provided.

These are some cases where a hydrostatic vent may be required in case of vertical pocket or vertical pocket with manual valve. Similarly, These are some cases where hydrostatic drain may be required

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

4 thoughts on “Importance of Vents and Drains in Piping System (With PDF)

  1. Hello Anup Kumar Dey,

    Well articulated and comprehensive article on Vents and Drains.

    More to be compiled and added on Automatic removal of Air and Dissolved Gases from Venting and at Drain points in STEAM operation Space inside pipelines and vessel .

    Also about Automatic Venting of Air from Cold and Hot Water piping systems in operation.

    Do share your communication IDs for proactive discussions, should you desire.

    Homi R. Mullan

  2. Hi , I am mech. Eng . We have a procedd piping L=90 m from : water , O2 , N2 , Ar .in horizontal arrangement . Where is best location for vent and drain connections ? If is nessacary redesign arranement to vertical ?

  3. Dear Mr.Anup Kumar,

    Due to height, no access to vent valve isolation. Is it possible to connect pipe to vent valve line and isolation valves installed at lower position, which is operation level on the ground?

    Please advise.

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