Report No FR0452

RELIABILITY AND IMPACT OF FAILURE AT
SEWAGE PUMPING INSTALLATIONS

FR0452

APRIL 1994

SUMMARY

I BENEFITS

A better understanding of the causes of pumping installation failures will lead to savings in implementing measures to reduce the frequency and impact of failures.

II OBJECTIVES

To assess the causes and effects of pumping installation failure and to give guidance on improved methods to minimise the frequency and impact of failure.

III REASONS

There are approximately 16,700 pumping installations in England and Wales. In the event of failure sewage may be discharged via an emergency overflow or backup and cause local flooding. The sewage discharged in the event of a pumping installation failure may be undiluted dry weather flow. The concentration of pollutants and the resulting impact on a receiving watercourse is potentially much greater than the discharge from a combined sewer overflow operating in wet weather conditions.

IV CONCLUSIONS

Information has been collected on the frequency and types of failures at over 300 pumping installations over a period of 6 to 24 months. This represents information from 1.7% of all sewage pumping installations in England and Wales.

The data indicates that there are approximately 120,000 recorded failures or partial failures of pumping installations per year in England and Wales, but only 2% of these (2,500) result in discharges to watercourses or flooding. Existing measures to limit the impact of failures are effective in 98% of all cases.

The most common causes of failure and discharge/flooding incidents are:

  1. pump failures - which represent 42% of failures and cause 41% of discharge/flooding incidents;
  2. rising main failures - which represent 1% of failures and cause 32% of discharge/flooding incidents;
  3. control failures - which represent 16% of failures and cause 17% of discharge/flooding incidents; and
  4. Power failures - which represents 33% of all failures but only cause 4% of discharge/flooding incidents.

A high proportion of discharges occur in wet weather and are therefore partly diluted by stormwater.

The low level of discharges and flooding incidents arising from public power supply failures is achieved without the widespread use of permanently installed standby generators.

Rising main failures cause a significant proportion of discharge/flooding incidents. Asbestos cement and PVC rising mains have significantly higher risk of failure than iron mains.

Current design guidance is predicting significantly reduced fatigue lives for PVC mains compared to the guidance given in British Standard CP312 which was last revised in 1977.

V RECOMMENDATIONS

Measures to reduce the impact of pumping installation failures should concentrate on those types of failure which cause most discharge/flooding incidents.

The provision of measures to prevent or limit failure at sewage pumping stations should be based on an objective assessment of risk and consequences of failure.

Policy on the provision of permanent standby generators should be reviewed and their use restricted to installations where the risk and consequences of failure cannot be contained by other means.

The design of PVC and polyethylene rising mains should take account of fatigue life under surge pressure.

Rising mains of types which are at higher risk of failure (e.g. PVC) should be assessed to identify whether measures are needed to extend the life of the main or whether replacement is required.

VI RESUME OF CONTENTS>

This report looks at the causes and consequences of sewage pumping installation failure and considers the methods available reduce failure frequency and minimise environmental impacts of failures. Details of a survey of failures of over 300 pumping stations are presented and conclusions drawn. The main causes of failure are discussed. Public power supply failure and rising main failure are discussed in more detail.

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members.