May 2020

Valves, Pumps and Turbomachinery

Causes of reduced or stopped flow in controlled-volume pumps

Controlled-volume (CV) pumps, also known as metering or dosing pumps, are designed to deliver a small quantity of fluid at a precise rate with relatively high pressure.

Controlled-volume (CV) pumps, also known as metering or dosing pumps, are designed to deliver a small quantity of fluid at a precise rate with relatively high pressure. They are essentially a positive displacement pump of the reciprocating design, but with a more complex design that prevents product leakage. CV pumps are used in various applications to inject solutions or chemicals, such as corrosion inhibitors, acids and caustics, into process streams.

Although these pumps are small in size and power and have a relatively simple operation principle, determining the cause for a reduction or stoppage in flow can be difficult. In this article, four different cases of pump failures are discussed where the causes of these failures were found to be due to liquid characteristics, pump and system design, or pump maintenance.

Pump design and operating principle

A CV pump (FIG. 1) consists of two sections: a driver section and a liquid section. The driver section includes gearbox assembly, which moves a plunger axially through a connecting rod at a reduced speed. This movement is called a stroke, which can be adjusted to control pump capacity.

Fig. 1. Main sections of a CV pump.

The liquid section simply consists of the liquid cavity locked between one-way valves (check valves), where one is a suction check valve and the other one is a discharge check valve. Some designs have more than one check valve in series for both suction and discharge.

The two sections are separated by a leak-free diaphragm made out of flexible material, such as Teflon, rubber or a thin, flexible sheet of stainless steel. This diaphragm reciprocates back and forth, synchronizing with the plunger movement. The mechanism of driving the diaphragm can be mechanical (linkages attached to the plunger and diaphragm) or hydraulic, where a hydraulic fluid fills in the cavity between the plunger and diaphragm (hydraulic chamber).

A complete movement of the plunger from one side to another is called a stroke. When the plunger moves away from the liquid side, it creates a vacuum that causes the suction check valve to open and pull pumped fluid inside the liquid cavity. This is called a suction stroke.

In contrast, the reverse movement of the plunger will add pressure to the enclosed volume, causing the discharge check valve to open and push the liquid outside (discharge stroke). This cycle is continuously repeated to deliver a certain amount of volume at a certain rate. The flowrate can be controlled either by adjusting the stroke length (distance the plunger travels), controlling hydraulic fluid volume inside the hydraulic chamber or adjusting the number of strokes (variable speed drive).

All CV pumps have the same operating principle, as previously explained. Based on each pump’s specific design, other characteristics may be considered when investigating a reduction or stoppage of flow.

Case 1: Priming of pump liquid end side and pump system

Pump priming is essential before restarting any pump after maintenance on the pump or its piping system. As stated, CV pumps are especially sensitive in this perspective due to their small pumped capacity. Priming is required to release any trapped air by venting pump casing and system piping from the highest feasible points. Venting valves should be kept open until continuous liquid is seen flowing. If venting lines are directed to a closed system and the flow is invisible, then sufficient time for venting should be given to ensure that the pump is properly primed. In this case, two pump designs from two different vendors are considered.

The first pump does not include a vent on the head and has a spring-loaded discharge check valve, as shown in FIG. 2. Plant personnel were in the habit of opening a vent on the pump discharge line to release the trapped air; however, air was still trapped in the pump head since the pump discharge check valve was spring-loaded. Even when the discharge pipe vent was open, the pump discharge check valve remained closed, preventing air release. This was mainly due to the low suction pressure from the suction tank, which was much lower than the pressure needed to open the spring-loaded discharge check valve.

Fig. 2. Pump without casing vent and with spring-loaded check valve.

To overcome this issue, the venting was achieved by temporarily connecting the pump suction to a high-pressure source through a hose hookup. In this case, the pump vendor did not include the vent in the design, as the vendor considered it optional. Instead, the vendor added a note to the operating manual, stating that a high-pressure source may be needed. The project contractor did not add a permanent connection for a high-pressure source at the suction. As such, a temporary house hookup was used every time the pump and system piping needed priming.

As an alternative solution, the pump head can be modified to add a casing vent. Another solution is to remove the spring from the discharge check valve, along with the process check valve. The main reason for the spring-loaded check valve is to provide enough force for closing the valve and preventing liquid from returning from the discharge pipe into the pump cavity. This needed force can be obtained from the process backpressure if the process check valve is removed. This way, the entire system can be vented when the discharge isolation valve is closed and the discharge vent connection is open, since the suction pressure will be higher than the check valve’s closing force.

For the second pump design, the pump casing included a vent for the pump head as part of the double discharge check valve, as shown in FIG. 3. In addition, it is worth mentioning that the discharge pipe of each pump was designed with a check valve (also called the process check valve).

Fig. 3. Pump with a casing vent and spring-loaded check valve.

In this case, the pump and suction line were vented through the pump vent connection. The pump started, but did not build any pressure since the discharge line was not primed. Even after opening the discharge isolation valve, the piping section from the pump check valve to the process check valve remained at low pressure and was not be primed. As a result, the pump required a long time to fill and build pressure in this pipe section. It is important to ensure that the discharge pipe is filled as part of the startup procedure, to ensure quick pressure buildup once the pump starts.

As an alternative solution, the process check valve can be removed so that every time the isolation valve is opened, the discharge pipe pressure will be at the same pressure as the main header. On the same subject, the backpressure is not the only concern with the piping system. The pumps with spring-loaded discharge check valves will not have any problem building up pressure inside their cavities. The time required for the pump to fill the empty portion of the discharge pipe can be long, considering pump low capacity. This depends, of course, on the pipe diameter and length to the process check valve.

For example, if the pump flowrate is 2.5 l/hr (which means it will deliver only 0.042 l/min), then the required time to fill up a 118-in. pipe with a 0.5-in. diameter is about 10 min. This example shows the approximate time before the discharge pressure is high enough to open the process check valve. Normally, the operator will immediately suspect a pump failure when the pressure gauge is reading low pressure. For this reason, the process check valve is recommended to be removed in such cases.

Case 2: Priming of hydraulic chamber

It is always recommended to follow the pump manufacturer’s instructions for the priming and startup of a pump. Startup procedures and precommissioning checks can differ from one manufacturer to another, depending on the pump design and internal details.

One important aspect is how to vent the pump hydraulic chamber. It is critical to ensure that the hydraulic section is vented properly and not rely only on the oil level indication. The hydraulic chamber is a contained cavity between the pump plunger and the diaphragm, as depicted in FIG. 4. This chamber is connected to an oil reservoir through internal passages.

Fig. 4. Effect of air bubble inside CV pump cavity.

Different mechanisms exist to control CV pump strokes. In some designs, continuous oil exchange between oil reservoirs and hydraulic chambers is used to control pump flow duty. (Note: Hydraulic fluid volume varies based on adjusted capacity.) For other designs, hydraulic oil volume is fixed, and capacity is controlled by adjusting stroke length. (Note: The oil reservoir feeds the chamber only when hydraulic oil is reduced.)

Regardless of the mechanism, the hydraulic chamber should be properly vented following the vendor instructions. Since the displaced volume of this type of pump is very small, a small amount of air can have a huge impact on pump performance.

To further explain this impact, the following parameters are used as an example (Eqs. 1–2):

          Pump capacity (Q) = 1 gal/hr = 0.0167 gpm = 0.0632 l/min                       (1)

If the pump stroke speed (S) is 110 stroke per min (spm), then the displaced volume per stroke is:

          Vd = Q / S = 0.0632 / 110 = 0.57 ml/stroke                                              (2)

If we assume a bubble of 1 ml (V1) is inside the hydraulic chamber (P1 = Patm = 14.7 psia) and the pump (fully primed) is started with a backpressure of only 45 psia from a spring-loaded check valve, then the pump operation will result in compressing the bubble size during the discharge stock. The change in the bubble size is described in Eq. 3:

          P1V1 = P2V2V2 = (P1 / P2)V1 = 0.33 ml → V = 0.67 ml > V               (3)

As shown, CV pumps are designed for a small flowrate, which means that the displaced volume per stroke is very small. When the variation in bubble volume (V1V2) is larger than the displaced volume per stroke (Vd), then all the work done by the pump is used to compress the bubble, and with no outflow from the pump (i.e., no pumping). In the previous example, a small backpressure (from only the spring-loaded discharge check valve) is assumed, as the pump is pushing the liquid to an empty discharge pipe during startup. Backpressure is much higher during normal operation, which also makes the difference in volume higher. In other words, pump performance can be impacted even with a smaller bubble volume than assumed in the previous example.

Case 3: Priming of cavity between pump double diaphragms

For hazardous applications or when process leakage must be prevented, a double-diaphragm design is specified to add a medium cavity between the liquid and hydraulic sides. When one of the diaphragms is ruptured, this cavity between the diaphragms will be filled with either process media or hydraulic oil, and the rupture can be detected by installing a pressure detection or liquid sensor.

Under normal operation, these two diaphragms have a synchronized movement. The cavity between the two diaphragms is usually vacuumed to ensure that they are in contact during operation. If this is not done during pump assembly at the shop, then the cavity will be filled with air. As a result, the cavity will be compressed during the pump stroke, preventing full power transmission to the liquid side and affecting pump performance (similar to Case 2). Therefore, it is important to remove the air between the two diaphragms, either by vacuuming the cavity or adding oil between the two diaphragms during pump assembly to ensure air removal and synchronized movement of the diaphragms (FIG. 5).

Fig. 5. Double-diaphragm pump with leak detection.

Case 4: Considerations based on pumped liquid

Liquid characteristics and chemical composition can impact pump performance. As this discussion focuses on the importance of proper priming, it is worth addressing the impact of process liquid, such as sodium hypochlorite (NaClO), on CV pump priming.

NaClO is commonly used in water treatment plants and is known for natural decomposition, especially at warm ambient temperatures, which results in generating oxygen bubbles. Continuous accumulation of these bubbles can “de-prime” the pump and cause a drop in performance. Since this is a characteristic of the NaClO solution and cannot be controlled, the issues associated with pumping this product should be considered during pump selection and system design.

As mentioned, temperature is a key factor that affects bubble generation rate. Therefore, for the suction tank and the suction piping, a sunshade and/or insulation can help reduce generated heat. Suction piping can also be designed with proper sloping so that generated bubbles can be redirected back to the suction tank.

Moreover, as discussed in Case 2, pump size and capacity can improve performance. Increasing pump capacity will result in bigger liquid-end and hydraulic-end cavities and, therefore, a larger displaced volume per stroke. The pump can tolerate some bubble accumulation to be pushed through the pump. Of course, the required dosing rate is determined by process requirements, but the point is to emphasize the importance of proper equipment sizing by selecting a pump with a high volume per stroke and a low number of strokes. Pumps with small flowrates per stroke in this application can be problematic.

Finally, plant personnel should be aware of the special characteristics of this solution. As frequent venting may be required, a pump casing vent is essential for such applications. HP

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