NET POSITIVE SUCTION HEAD (NPSH):

Net Positive Suction Head, or NPSH, can be defined as two parts:

• NPSH Available (NPSHA): The absolute pressure at the suction port of the pump. NPSHA is a function of your system and must be calculated.

AND

• NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSHR is a function of the pump and must be provided by the pump manufacturer.

IMPORTANT: You must have more suction side pressure available than the pump requires.

NPSH STARTING VALUE RULES OF THUMB:

There are two forms of Net Positive Suction Head (NPSH). NPSHA or NPSH (available) is the resulting pressure value of the liquid entering the pump. NPSHR or NPSH (required) is the resulting pressure drop produced as the liquid passes through the pump. NPSHA is defined by the system within which the pump operates; it must be calculated. NPSHR is a characteristic of the pump itself; it is determined from tests conducted by the pump manufacturer.

The greater the margin is between NPSHA and NPSHR, the better the pump will perform.  A pump station design that has taken into account the proper calculations and maximizes NPSHA will be far superior to a pump station that has not been designed, with these factors in mind.

WELL SITE CONSIDERATIONS:

WELL IMPOUND AREA SIZE AND DEPTH: Proper Transitions from the water impound area to the pumping station:

PEAK SUMMER TEMPERATURE:

The (Peak Summer Temperature) is a critical factor when selecting your Pump Station. As the Air Temperature increases or decreases so does the Atmospheric pressure. Atmospheric pressure changes will have a direct effect on the performance of your Pump Station. (Higher temperatures = less density = less pressure).

Pump Selection Basics

ACCEPTABLE PUMP CURVE SHAPE:

A pump curve shape should be a consistent curve throughout the required operating level for your pump station. A pump curve that is flat throughout the desired operating level can be hard to control when small changes of system resistance occur.  Your pump should be selected to operate at max efficiency at your system design level.

The example below shows a pump designed to operate at 300 GPM @ 231’ TDH. (100PSI):

The black line represents the curve for the pump. This curve is consistent throughout the desired operating range.

The red lines represent the desired flow rate. Where the red lines intersect and meet the black line (pump curve) shows this pump has been properly selected to meet the demands of 300 GPM @ 231’ TDH. (100PSI).

BEST PRACTICES FOR SUCTION DESIGN:

Consideration must be taken for Maximum flow run out and mitigation strategies.

If the demand on the pump has the possibility of overloading the pump beyond the design point, such as power outages or frequent system restarts, then the maximum pumping capability must be considered, and the intake piping and inlet devices must be calculated and sized correctly in the design process.

The danger is that the pump will overload and break suction or cavitate and potentially damage the pump incrementally as it goes in and out of its optimum design point or as temperature and barometric pressure changes.

MULTI PUMP SUCTION LIFT STRATEGIES:

In multi-pump design whenever practical individual pump suction design is simpler and less problematic.

Shared suction lines on multiple pumps have the tendency of the operating pump drawing water away from the ideal pump and it will then attempt to start and run dry and breaking suction on start or partially dry and cavitating as it come up to speed.

Multiple strategies can be employed with varying levels of success to mitigate this problem.

PROPER INTAKE SCREEN CAPACITY SELECTION / MESH SELECTION:

Selecting the proper Intake screen is critical to the operation of your Pump Station. If the proper screen size/type is not used it will starve water to your pump.

• The proper Intake Screen will ensure a steady supply of clean water to your pump which ensures pumping efficiency and lower maintenance costs with your Pump Station.
• Selecting the proper sized Intake Screen is based off the desired flow of your Pump Station.
• Use the chart below to determine the correct Intake Screen for your Pump Station:

FOOT VALVE SELECTION AND SIZING:

A foot valve is a form of a check valve that is usually in a vertical position and has a screen incorporated. The foot valve will be installed onto the end of the pump station suction line. The purpose of the foot valve is to preclude back-flow from the pump through the suction line. They are installed on the end of the suction line to help the pump maintain its prime. By necessity though they create an obstruction to the flow of water to the pump so some friction loss will need to be factored in.

Selecting the correct Foot Valve is based off your suction line diameter and the style needed for your application. There are various options when it comes to selecting the proper foot valve for your application. The main difference is the material of construction and the diameter of the screen. The quality of water will be a determining factor when selecting which screen size to use.

PIPE SIZE / MATERIAL / ROUGHNESS / ID:

Selecting the proper Pipe Diameter and material is based off the following factors:

1. The GPM and Pressure of your Pump Station Design. The Feet per Second should be under 10 when selecting the correct pipe diameter. FPS is the velocity of the water going through the pipe.
2. The quality of water and the environment that the Pipe will be exposed to. Water has different levels of acidity that will affect the life span of the pipe. This information is critical when selecting the correct pipe material. If your pump station is exposed to the elements it may require the pipe to made from specific material that will hold up to the elements. Example: Saltwater environments are very corrosive.
3. The amount of pressure a pipe material can handle. Different materials and grades of those materials are designed to handle different pressure ranges. Determining your Pump Station design pressure is a critical factor in selecting the right material.

TROUBLE SHOOTING SUCTION LIFT PUMPING SYSTEM / RECOGNIZING CAVITATION:

1. Noise and Vibration– Usually a rattling sound with frequencies from 10 kHz to 100 kHz. The noise indicates cavitation is happening and in most cases will cause damage to the pump or piping.
2. Surface Damage– One of the most obvious signs of cavitation can be found by inspecting your impeller. When cavitation occurs, it causes tiny bubbles in the water which collapse. When these cavitation bubbles collapse and encounter a solid surface, they will cause flaking of the material. This over time can damage your impeller.
3. Pump Performance- When there is significant cavitation it will reduce the available power of your pump. In turn there will also be a noticeable head loss. Cavitation alters the pressure distribution around the impellers blades which has adverse effects on the performance of your pump. A drop in pump performance and head loss over time is a good indication of cavitation issues.

TROUBLE SHOOTING AIR LOCKS:

If you can hear your pump running, but it is not pumping water, it is likely air-locked. The following are steps to take when trouble shooting air locks in your system.

• Check to make sure there is available water to the pump
• Check to make sure the correct valves are open/closed

How To Use The NPSH Friction Loss Calculator:

(See input explanations and instructions below)

3) FLOW RATE:

The GPM of your pump station in gallons per minute. The pump selected should run at max efficiency at the desired system design, but also should be able to handle small changes of flow rate that can happen while operating. The pump selected should have adequate capacity to handle these changes.

5) PIPE SIZE:

Nominal pipe size in inches

7) PIPE SCHEDULE OR HDPE DR:

Enter pipe schedule or HDPE DR here. If you get a “NO MATCH” error on Pipe ID, that schedule is not available for the pipe size entered. Select the closest schedule until you get a match.

8) PIPE MATERIAL:

Select the type of pipe material to be used in your pump station.

9) FITTING TYPES AND COUNT:

Add fittings and quantity of each to be for friction loss calculation. First select fitting type from drop down menu. Then select the quantity of each fitting. Then click the ADD FITTING button.