Some Utility Consumption Estimates

As part of preparing a process discipline manual for one of my previous organizations, one section that I had prepared was "Guidelines for Preparation of Utility Summary". This document besides containing an excel template for utility summary also had some guidelines for determining the consumption of utilities such as instrument air, cooling water make-up, potable water and water requirements for a safety shower. I am reproducing some information:

1 Instrument Air Requirement
The following guidelines may be adopted for estimating instrument air consumption for different types of pneumatic instruments:

- Control Valves with diaphragm actuator and positioner and I/P converter-2.0Nm³/h
- Control (generally on-off type) valves with piston actuator and positioner – 5 Nm³/h 10% of this consumption will be considered in calculations.

The estimated total instrument air quantity should be multiplied by 1.3 to account for uncertainties in the data used for the estimate and to allow the installation of additional instruments during the early years of plant operation.

2 Cooling Tower Make-up

-Evaporation Loss: Approximately equal to 1% of circulation rate for each 5.5°C (10°F) of cooling tower temperature range.
- Drift Loss: 0.1 % of circulation rate as a preliminary estimate.
- Blowdown: This quantity is dependent on circulation rate, range, and cycles of concentration

Total Make-up water = Evaporation Loss + Drift Loss + Blowdown

3 Potable Water Requirement

As a conservative estimate a figure of 150 litres per day per person should be considered for preliminary calculations for potable water. This includes water for drinking and sanitation.

4 Safety Shower

Water quantity requirement for safety showers to be considered based on each safety shower operating for 15 minutes per day. A minimum flow rate of 4.5 m³/h for each safety shower is recommended as per ANSI Z538.1. As an example if there are 10 safety showers required to be installed for the project, then the water requirement may be calculated as follows:

Quantity required per day per safety shower = 4.5 x (15/60) = 1.125 m³/day
No. of showers = 10
Total water quantity for safety showers for the project = 10 x 1.125 = 11.25 m³/day

Other utilities such as Fuel Gas, Nitrogen, Steam etc. are not considered because they cannot be generalized and require specific consumption calculations depending on the project / plant requirement.

Let me hear comments from the knowledgable members and readers of "Cheresources".

Regards,
Ankur.