2.3.12 Pump Operational Hazards and Risk

Centrifugal pumps mostly used in the pipeline industry are for transportation of crude (including heavy) oil — a mixture of liquid hydrocarbons, some with volatile at atmo­spheric conditions I such as condensate. Crude (particularly heavy oil) can be also warm (60-80°C).

Crude oil can also contain contaminants, some toxic such as hydrogen sulphide, radio­active such as strontium salts, or just water or grit. Additives may be introduced which have toxic properties, although these would normally be only in low concentrations. Therefore, hazard and risks associated with the pump operations must be considered and taken care of in the design and selection stage as much as possible.

The hazards associated with a large centrifugal pump must be considered over its com­plete operating/maintenance cycle. Operating range includes any abnormal and transient situations and loads. Poor operation/excursions/drive system failures and emergencies also are required to be covered.

The majority of hazards relate to the liquid being handled, either by a direct release, or by the consequent effects on upstream and downstream systems from a pump failure:

• failure of static components through fatigue, erosion or corrosion

• failure of dynamic components leading to high fatigue loads on other components with potentially rapid catastrophic deterioration of seal or nozzles

• failure of the piping system due to extreme pressures or temperatures either externally applied or generated by the operating pump or system, resulting from events such as pressure surges, process density, or liquid property changes

The direct threats to personnel from a release can arise from:

• the possible flammable nature of fluid released — volatile/high vapor pressure com­ponents, which would form a gas cloud with potential for explosion, or fire

• physical injury from a jet of fluid, or slips / falls from contaminated floor surfaces

• liquid which may well be hot, giving a scalding risk above 60-70°с range

• the liquid which may also evolve asphyxiating gases

• the toxic nature of components or additives within process material (liquids соntain hydrogen sulphides are highly toxic)

• small traces of radioactive salts within the process material can accumulate within a pump requiring appropriate handling precautions

• inappropriate operation of the pump can induce high temperatures and pressures within a pump, giving rise to hazards from mechanical disintegration of the pump

• handling material with higher concentrations of water or solids which can lead to higher pressure generation due to the effective increase in density

Mechanical Hazards from improper pump operation could include:

• dynamic instability — vibration leading to bearing and pump component dame

• liquid induced vibration — running outside bep

• over-speed and reverse rotation — mechanically or process-induced

• internal clearances

• bearing failure/lubrication

• seal failure

• joint failure

• loss of piping and pump restraints

• corrosion

• erosion

• failure of connections — overload and fatigue issues

• entrapment from contact with rotating shaft or other similar components

Pumps are designed and selected to operate at an optimum flow rate, i.e., minimum hydraulic loads and maximum efficiency. For operating pumps below a minimum flow rate that is specified by the manufacturer (typically 25% of optimum), the hydraulic loads generated can exceed the pump design conditions, induce flow instabilities, and cause over heating. This particularly manifests if there is not enough flow to carry away the ion energy.

Generally it is normal practice to start pumps at fully primed condition with suction valve open and discharge valve closed. This configuration will utilize minimal power, letting the driver run up to speed quickly. The discharge valve is then opened slowly to avoid pressure surges. Delayed opening of the discharge valve can heat the liquid, potentially to high temperatures, but any ensuing high pressures can be vented back through the suction valve.

No pump should ever be throttled by the suction valve, as this is likely to create cavitation and damage the pump. Cavitation process can also cause micro cracks on vulnerable components such as the pump impeller. Severe cases result in loss of performance due to internal pump damage.

While it is possible to throttle on the discharge valve, on a large or high head pimp, this is likely to damage the valve over time, and tight closure of the valve will then note possible. However this is the preferred method of controlling most pumps with fixed speed drivers.

Starting a pump with the delivery valve wide open takes the maximum amount oil power and may overload an electric motor driver if used. In addition, if the pump is delivering into empty pipework with a closed valve at the far end, "surge pressure" effects can produce a shock wave, potentially doubling the discharge pressure, damaging instrument™ and displacing equipment.

An abrupt loss of the steady operation of the pump may cause disruption of the f I stream or downstream systems. This may result in systems being shut down or load being transferred onto other pumps. The pump stop will cause "surge pressure" effects and a "negative" pressure wave. This can cause water hammer, particularly in long pipelines.

Chapter 3 DESIGN AND OPERATION OF POSITIVE________________ DISPLACEMENT PUMPS______________________________

The most commonly used pumps for trunkline pipeline stations are either centrifugal or positive displacement (reciprocating and rotary pumps) (Fig. 3-1). As these pumps have the most prevalent application in pipeline transmission, their theory and application are detailed in the chapter. Pipelines handling liquids in excess of about 110 cS (centistoke) vis­cosity will normally use rotary, positive displacement pumps both for optimum efficiency and, frequently, lower initial cost. Centrifugal pumps are generally high-speed, high-volume units connected through speed increasers to internal combustion engines or directly to electric motors. Centrifugal pumps offer certain distinct advantages; chief among these is the fact that the flow of liquid from them is relatively even and smooth with very few pulsations. Prop­erly installed and operated, little or no vibration results from their use. These pumps can be used outside or in small buildings, need only light foundations, and are easily kept clean. In addition, these pumps are of comparatively low cost (capital and operation), are simple to construct and flexible to operate, and require a comparatively small space. Positive displacement pumps are divided into two general classes: rotary and reciprocating. These are described in detail below