Pump characteristic curve
Head (H) of the pump - overpressure created by the pump. The head is measured in (m).
The head that the pump must provide is the sum of the geodetic height difference and head loss (= height of losses) in pipelines and fittings.
It should be borne in mind that when starting, and then during operation, the pump changes its operating mode. The choice of pump motor power should be carried out from the conditions that it operates at the maximum load in a certain period of time, for example, at H geo max. Let's consider how this value varies depending on the mode of operation of the pump.
Let's consider an example: the pressure pipeline is laid on a variable terrain and has several peaks. At start-up, when the discharge line is empty, the pump should raise water from the NN level (-1 m) to the height NN1 (10 m), and after filling the pipeline NN1-NN2, it must raise the water to the NN3 height (11 m).
At the initial time to fill all sections of the pipeline, the pump must overcome the height Hgeo max, equal to:
Hgeo max = (NN1-NN) + (NN3-NN2)
= + (11 m - 5 m)
= 17 m
When the pipeline NN-NN 3 is filled with drains, the geodetic height decreases:
Hgeo = NNA - NN
= 6 m - (-1 m)
= 7 m
Comments on the calculation of geodetic heights:
If air is not removed from the pressure line, then The geodetic height is defined as the sum of the heights of all the ascending pipelines (plot 1 + plot 3), since in this case additional energy is expended on compressing air in the descending section (plot 2). Therefore, it takes a lot of energy to overcome the high-rise points.
When the pump is operated without removing the air from the pressure line: after the air is expelled from the pipeline, the pipeline is filled completely. Therefore, the pressure that the pump must provide is determined only by the geodetic elevation Hgeo between the output / transfer reserve NNA and the water level in the NN shaft, at which the pump is shut off.
If air is removed from the pipeline, then when the pump is turned on the difference between the water level in the shaft (pump start point) and the highest point Hgeo max should be taken into account.
When operated with air removal: during operation, the pump operates in the same mode as "without deaeration".
For the correct choice of the pump and the motor, it should be taken into account that they can operate in different modes. This must be done to prevent the pump or motor from failing and guarantee their optimum performance.
Determination of the concept of pressure
Pump characteristics form. The different steepness with identical housing and pump impeller (for example, depending on the engine speed)
Various feed and pressure changes
Pump head (H) - specific mechanical work transmitted pump pumped fluids.
H = E / G [m]
E = mechanical energy [N m]
G = weight of pumped liquid [N]
The pressure created pump , and the flow rate of the pumped liquid ( innings) depend on each other. This relationship is displayed graphically in the form of a pump characteristic. The vertical axis (ordinate axis) reflects the head the pump (H), expressed in meters [m]. Other scale scales are also possible. The following relations are valid:
10 m. = 1 bar = 100,000 Pa = 100 kPa
The horizontal axis (the abscissa axis) shows the scale of the pump (Q), expressed in cubic meters per hour [m3 / h]. Other scale scales are possible, for example [l / s].
The form of the characteristic shows the following types of dependence: energy electric drive (taking into account the general Efficiency) is converted to the pump in such forms of hydraulic energy as pressure and speed. If pump It works with the valve closed, it creates the maximum pressure. In this case, we speak of the head of the pump Ho at zero feed. When the valve starts to slowly open, the pumped medium comes into motion. Due to this part of the drive energy is converted into the kinetic energy of the fluid. Maintaining the initial pressure becomes impossible. The characteristic of the pump takes the form of a falling curve. Theoretically, the pump characteristics intersect with the feed axis. Then water has only kinetic energy, that is, pressure is no longer created. However, since there is always an internal resistance in the piping system, in reality the characteristics of the pumps are cut off before the feed axis is reached.
How to calculate the head of the well pump?
When it comes to pump parameters, then there is one hard question, which is difficult answer is easy!! Most experts from this field answered - what a complex and controversial question of computation exists.
You are lucky!!! We found an easy algorithm for calculating the pump head. According to this calculation, even a person far from this, can make a calculation.
And so, let's get started! The image shows a diagram that allows you to see all the pressures necessary for calculations. If there is no image, refresh the page! If the image does not appear after updating the page, write in the comment about this problem. Comments are at the end of the article.
Scheme 1 (see image):
At first, who is far from understanding the "pressure", I recommend that you read: What is the pressure?
Secondly, a short story about how to understand this scheme. The scheme has vertical dimensions. It is necessary to cut off all the pipeline turns - they do not exist. In addition, the computation of the calculation of the head of the pump does not affect the horizontal. All you need to know is their altitude. Since in most cases the length of horizontally located pipes is very small and does not exceed 30 meters. 30 meters for very little and not worth it to pay them into account. The difference does not exceed 10%. And also the horizontal pipeline adds only dynamic hydraulic resistance. And for the calculation we need to know only the head created by the height.
Thirdly, Pressure and pressure synonyms. Pressure in 1 bar = 10 meters of head.
To be more precise:
For calculation First of all you need to know or choose the right pressure in the crane of the last (second) floor. For a private house, you can take 10 to 25 meters of head. For example, in the central, in apartments this redistribution is from 20-40 meters. 10 meters is considered an economical option and is quite suitable for water supply. In addition, the lower the head, the more you save the energy consumed by the pump - a fact!
For example, remember, come in handy: I choose 10 meters of minimum head in the crane of the second floor.
Accordingly, the minimum head in the floor 1 crane will be equal to the height difference. If the floor is 3 meters, then the minimum head = 13 meters.
The automatic block of the pressure switch is from the tap of the second floor at a height of 6 meters below. This means that the minimum pressure in the pressure switch is 16 meters. And so the pump start threshold will be 16 meters (1.6 bar).
Add to 16 meters another 15 meters and get the maximum pressure of the relay. That is, the trip threshold of the relay must be 31 meters. You can, of course, for economy and add 10 meters. And then get the cut-off threshold of 26 meters and also be right.
And so we choose a pump cutoff threshold of 31 meters.
To find this pressure you need to know:
Most experts will not immediately tell you the minimum water column. The minimum column of water is determined practically. If you do not have such data, you can safely take into account the minimum height of the pump from the bottom. That is, in our case, we take the minimum column of water where the pump is located (pump top point).
Data for our case:
Calculation: Set pressure = Depth of well - minimum water column + height (from ground to relay automation) + Maximum pressure of pressure switch. Total = 30 - 10 + 2 + 31 = 53 meters.
A little training in the calculations, you can think differently - it's easier. We need to know the height from the minimum water column to the maximum head for calculation. See the diagram above.
1. To make up for losses when the mains voltage drops. When the voltage in the network drops, then the head of the pump itself falls.
2. For good achievement of the maximum pressure threshold. If you pick up with the pressure set, the situation may arise when the pump can not reach the maximum pressure threshold on the relay. And the system will hang in suspense. Very often in this case, the pumps are burned. If your voltage is weak, then put voltage regulators.
Determination of the concept of pressure
Increasing pressure by a pump is called a head. Under the head of the pump (H) is meant the specific mechanical work transferred by the pump of the pumped liquid.
H = E / G [m]
E = mechanical energy [N m]
G = weight of pumped liquid [N]
In this case, the pressure created by the pump and the flow rate of the pumped liquid (feed) depend on each other. This relationship is displayed graphically in the form of a pump characteristic. The vertical axis (ordinate axis) reflects the pump head (H), expressed in meters [m]. Other scale scales are also possible. The following relations are valid:
10 m. = 1 bar = 100,000 Pa = 100 kPa
On the horizontal axis (the abscissa axis), the pump feed scale (Q) is plotted, expressed in cubic meters per hour [m3 / h]. Other scale scales are possible, for example [l / s]. The form of the characteristic shows the following types of dependence: the energy of the electric drive (taking into account the overall efficiency) is converted into a pump in such forms of hydraulic energy as pressure and speed. If the pump is operated with the valve closed, it generates the maximum pressure. In this case, we speak of the pump head H 0 at zero feed.
When the valve starts to slowly open, the pumped medium comes into motion. Due to this part of the drive energy is converted into the kinetic energy of the fluid. Maintaining the initial pressure becomes impossible. The characteristic of the pump takes the form of a falling curve. Theoretically, the pump characteristics intersect with the feed axis. Then water has only kinetic energy, that is, pressure is no longer created. However, since there is always an internal resistance in the piping system, in reality the characteristics of the pumps are cut off before the feed axis is reached.
- Different steepness with identical housing and pump impeller (for example, depending on engine speed)
Pump characteristics form
The figure shows the different steepness of the characteristics of the pump, which may depend, in particular, on the speed of the motor.
In this case, the steepness of the characteristic and the displacement of the operating point also affect the change in flow and pressure:
flat curve
- greater feed variation
with a slight change in head
steep curve
- large feed change
with a significant change in pressure
The friction occurring in the pipeline network leads to loss of pressure of the pumped liquid along the entire length. In addition, the pressure drop depends on the temperature and viscosity of the fluid being pumped, the flow rate, the properties of the reinforcement and the aggregates, and also the resistance due to the diameter, length, and roughness of the pipe walls.
The loss of pressure is displayed on the graph as a characteristic of the system. For this, the same graph is used as for the pump characteristic.
Characteristics of the system
The form of the characteristic shows the following dependencies:
The cause of the hydraulic resistance occurring in the pipeline network is the friction of water against the pipe walls, the friction of the water particles against each other, and the change in the flow direction in the fittings of the reinforcement.
When the feed is changed, for example, when opening and closing the thermostatic valves, the flow velocity and thus the resistance also change.
Since the cross section of the pipes can be considered as the area of the live section of the flow, the resistance varies quadratically. Therefore, the graph will have the form of a parabola. This relationship can be represented as the following equation:
H1 / H2 = (Q1 / Q2) 2
conclusions
If the supply in the pipeline network is halved, the head drops by three quarters. If, on the contrary, the supply increases twofold, then the head rises fourfold. As an example, you can take the flow of water from a separate faucet.
With an initial pressure of 2 bars, which corresponds to the head of the pump approx. 20 m, the water flows out of the DN 1/2 valve at a flow rate of 2 m3 / h.
To increase the supply by half, it is necessary to increase the initial inlet pressure from 2 to 8 bar.
Operating point
The point at which the characteristics of the pump and system intersect is system operating point and pump. This means that at this point there is an equilibrium between the useful power of the pump and the power consumed by the pipeline network. The pump head is always equal to the resistance of the system. This also depends on the supply that can be provided by the pump.
It should be borne in mind that the feed must not be below a certain minimum value. Otherwise, this can cause too high a temperature rise in the pump chamber and, as a result, damage to the pump. To avoid this, follow the manufacturer's instructions.
The operating point outside the pump's characteristic can cause damage to the motor. As the flow rate changes, the operating point is constantly shifted during the pump operation. To find the optimal design work point in accordance with the maximum operational requirements is included in the tasks of the designer.
Such requirements are:
- for circulation pumps of heating systems - consumption of heat by the building,
- for pressure boosting systems - peak flow rate for all draw-off points.
All other operating points are to the left of this calculated operating point.
The two figures show the effect of the change in the hydrodynamic resistance on the displacement of the operating point. The displacement of the operating point in the direction to the left of the calculated position inevitably causes an increase in the pump head. As a result, noise in the valves arises. The pressure and flow control according to the need can be made using pumps with a frequency converter. At the same time, operational costs are significantly reduced.
With the arrangement of water supply and heating of country houses and cottages, one of the most pressing problems is the selection of a pump. An error in the choice of the pump is fraught with unpleasant consequences, among which the overspending is the simplest, and the failure of a submersible pump is the most common. The most important characteristics for choosing any pump are the water flow or pump capacity, as well as the pump head or the height at which the pump can supply water. The pump is not an equipment that can be taken with a reserve - "for growth". Everything must be reconciled strictly according to the needs. Those who are too lazy to make the appropriate calculations and chose the pump "by eye", almost always there are problems in the form of failures and breakdowns. In this article, we will dwell in detail on how to determine the pump head and productivity, provide all the necessary formulas and tabular data.
Submersible pumps are usually installed in deep wells and wells, where the self-priming surface pump can not cope. Such a pump is characterized by the fact that it works completely submerged in water, and if the water level drops to a critical level, it turns off and does not turn on until the water level rises. The operation of a submersible pump without water "dry" is fraught with breakdowns, so it is necessary to select a pump with such a capacity that it does not exceed the debit of the well.
Calculation of the capacity / flow rate of the submersible pump.
The productivity of the pump is not in vain sometimes called the expense, since the calculations of this parameter are directly related to the flow of water in the water pipe. In order for the pump to be able to meet the needs of the tenants in the water, its productivity should be equal to or slightly more than the consumption of water from simultaneously switched consumers in the house.
This total expenditure can be determined by adding the costs of all, possibly simultaneously included, water consumers in the house. In order not to bother yourself with unnecessary calculations, you can use a table of approximate values of water flow per second. The table lists all possible consumers, such as a wash basin, toilet bowl, sink, washing machine and others, as well as water flow in l / s through them.
Table 1. Consumption of water consumers.
After you have summed up the expenses of all the required consumers, you need to find the estimated system consumption, it will be somewhat less, since the probability of simultaneous use of absolutely all sanitary appliances is extremely small. You can find the estimated flowrate from Table 2. Although sometimes, to simplify calculations, the total flow is simply multiplied by a factor of 0.6 to 0.8, assuming that only 60 to 80% of plumbing fixtures will be used at the same time. But this method is not entirely successful. For example, in a large mansion with a lot of plumbing fixtures and water consumers can live only 2 - 3 people, and the water flow will be much less than the total. Therefore, we strongly recommend using the table.
Table 2. Estimated consumption of the water supply system.
The result will be the real expense of the water supply system at home, which must be covered by the pump's capacity. But since in the characteristics of a pump the productivity is usually considered not in l / s, but in m3 / h, the flow rate obtained by us must be multiplied by a factor of 3.6.
Example of calculating the flow rate of a submersible pump:
Let's consider a variant of water supply of a country house in which there are such sanitary equipment:
- Shower with mixer - 0,09 l / s;
- Electric water heater - 0,1 l / s;
- Sink in the kitchen - 0,15 l / s;
- Wash basin - 0,09 l / s;
- The toilet is 0.1 l / s.
Summarize the consumption of all consumers: 0.09 + 0.1 + 0.15 + 0.09 + 0.1.53 l / s.
Since the house we have with a garden plot and vegetable garden, it does not hurt to add a watering cock, the consumption of which is 0.3 m / s. Total, 0.53 + 0.383 l / s.
We find the value of the design flow according to Table 2: 0.48 l / s corresponds to a value of 0.83 l / s.
We translate l / s in l / min, for this, 0.48 * 60 = 28.8 l / min.
And the last - we translate l / s in m3 / h, for this 0,48 * 3,6 = 1,728 m3 / h.
Important! Sometimes the pump capacity is indicated in l / h, then the resulting value in l / s must be multiplied by 3600. For example, 0.48 * 3600 = 1728 l / h.
Conclusion: the consumption of the water supply system of our summer cottage is 1,728 m3 / h, therefore the pump capacity should be more than 1,7 m3 / h.
To more accurately determine the appropriate pump model, you need to calculate the required head.
Calculation of the head of a submersible pump
The pump head or water lifting height is calculated using the formula below. It is considered that the pump is completely immersed in water, therefore parameters such as a difference in height between the water source and the pump are not taken into account.
Calculation of the head of the downhole pump
Formula for calculating the head of the downhole pump:
Where,
Htr - the value of the required head of the downhole pump;
Hreo - elevation difference between the location of the pump and the highest point of the water supply system;
Loss is the sum of all losses in the pipeline. These losses are associated with the friction of water on the material of the pipes, as well as the pressure drop at the corners of the pipes and in the tees. Determined by the loss table.
Hsvob - free pressure on the spout. In order to be able to comfortably use the plumbing fixtures, this value should be taken 15 - 30 m, the minimum permissible value is 5 m, but then the water will be supplied in a thin trickle.
All parameters are measured in the same units as measured in the pump head, in meters.
Calculation of losses in the pipeline can be calculated by studying the table below. Please note, in the loss table, in normal type, the rate at which water flows through a pipeline of the appropriate diameter is indicated, and in the selected type - the head loss for every 100 m of a straight horizontal pipeline. At the very bottom of the tables, there are losses in tees, corner joints, check valves and gate valves. Naturally, for accurate calculation of losses, it is necessary to know the length of all sections of the pipeline, the number of all tees, turns and valves.
Table 3. Loss of head in the pipeline of polymeric materials.
Let's consider such variant of water supply of the country house:
- The depth of the well is 35 m;
- Static water level in the well - 10 m;
- The dynamic water level in the well is 15 m;
- The well rate is 4 m3 / h;
- Well is located at a distance from the house - 30m;
- The house is two-storeyed, the bathroom is on the second floor - 5m high;
First of all, we consider Hreo = dynamic level + height of the second floor = 15 + 5 = 20 m.
Next, consider H to lose. We assume that the horizontal pipeline is made of polypropylene pipes 32 mm to the house, and in the house a pipe 25 mm. There is one angular turn, 3 non-return valves, 2 tees and 1 stop valve. The productivity is taken from the previous calculation of the flow rate of 1,728 m3 / h. According to the proposed tables, the nearest value is 1.8 m3 / h, so we will round it up to this value.
Loss = 4,6 * 30/100 + 13 * 5/100 + 1,2 + 3 * 5,0 + 2 * 5,0 + 1,2 = 1,38 + 0,65 + 1,2 + 15 + 10 + 1,2 = 29,43m ≈ 30m.
We will take 20m.
Total, the required pump head is:
Htr = 20 + 30 + 20 = 70 m.
Conclusion: Considering all losses in the pipeline, we need a pump whose head is 70 m. Also from the previous calculation, we determined that its capacity should be above 1,728 m3 / h.
Let's consider an example of selection of a submersible pump on the schedule "Water Jet".
At the intersection of 70 meters of head, and the required 30 liters per minute, a suitable pump will be "Vodomot" 60/92. The maximum water flow should not exceed the flow rate of the well, it should be 5-10% less than the well production rate. If this is not done, the operation of the pump will reduce the dynamic water level below the suction side of the pump. This is fraught with the operation of a pump without water - "dry running", which can still be avoided by putting the automation.
A more specific choice of pump is already dependent on the financial capabilities of the owner of the house.