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Featured Inquiry (08/07/09)Reviewed: 12/13

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Question:

What is the minimum allowable water pressure?

Answer:

In the September issue of Municipal Research News, we referenced the provisions of WAC 246-290-420(4) indicating that the minimum allowable water pressure at the customer's service meter must be no less than 20 pounds per square inch (psi) under MID conditions. However, one of our readers, correctly pointed out that WAC 246-290-230, which applies to new water distribution system construction, provides in part:


    (4) New public water systems shall provide a design quantity of water at a minimum pressure of 30 psi during MID conditions, and
    (5) Provide a fire flow at a minimum pressure of 20 psi during MID conditions.

This is a more restrictive requirement than WAC 246-290-420(4) which requires a minimum of 20 psi under MID conditions, excluding fire flow.

Therefore, existing water systems must provide a minimum water pressure of 20 psi at the customer's service meter, or property line under MID conditions per WAC 246-290-420(4); while all new water systems must be designed to provide a minimum water pressure of 30 psi at the service meter or property line per WAC 246-290-230.

MID refers to "maximum instantaneous demand," which means the maximum rate of water use, excluding fire flow, experienced or expected within a defined service area at any instant in time.

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