Anti-blocking pipeline for power plant
By installing auxiliary ash conveying units and unblocking units on the ash conveying pipeline, and utilizing pressurized gas and vibrators, the problems of blockage and wear in the ash conveying system were solved, achieving more efficient and stable dust conveying.
Patent Information
- Application Number
- CN202520475971.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-18
- Publication Date
- 2026-01-30
- Estimated Expiration
- 2035-03-18
AI Technical Summary
Existing ash conveying systems are prone to pipe blockage and severe wear, affecting boiler co-firing and coal costs. Furthermore, traditional conveying methods rely on high-pressure air, resulting in high dust flow rates and increased pipe wear.
Multiple auxiliary ash conveying units and auxiliary unblocking units are installed on the ash conveying pipeline. By using pressurized gas and vibrators, multi-point air delivery and physical vibration are achieved, reducing dust flow rate and the risk of blockage.
It effectively reduces the risk of ash conveying pipeline blockage, reduces dust flow rate, reduces pipeline wear, and improves conveying efficiency and stability.
Smart Images
Figure CN223851714U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of the power industry, in particular to a pipe for preventing blockage of a power plant. BACKGROUND
[0002] A dust conveying pipe for conveying dust from a dust collector of a thermal power plant to a designated ash storage is usually provided in a treatment system of a boiler combustion tail of a thermal power unit, and the dust conveying pipe and some systems associated therewith are collectively referred to as a pneumatic dust conveying system. Whether the pneumatic dust conveying system is safely operated or not is related to the stability of the boiler dust removal system. If the system fails, the high material level of the dust collector will be caused, and if the problem is not handled in time, the high pressure loop of the dust collector will be short-circuited, the dust collector will be unable to be put into operation, and the unit will be forced to reduce load or shut down. In addition, if a large area of the pneumatic dust conveying system leaks, it is easy to cause large-area secondary dust pollution. In order to avoid the occurrence of environmental events, the safe and stable operation of the pneumatic dust conveying system will produce great social and economic benefits.
[0003] The existing dust conveying system of a thermal power plant is mainly a simple bin pump fluidization system. The principle is that dust is added into the bin pump through a feeding port, the feeding valve is closed after the dust in the bin pump reaches a set amount, compressed air generated by an air compressor enters the bin pump through a fluidization plate to make the dust particles in the bin pump suspended and form a fluidized state, so as to reduce the internal friction and adhesion of the dust. The fluidized dust is driven by the compressed air pressure to be conveyed to a target ash bin through a dust conveying pipe, so as to complete the dust conveying task. In actual production, the annual average of the ash content of the burned coal is 16.15% (the upper limit is 22.08%), which is much higher than the design coal ash content of 14.56%. This will significantly increase the amount of fly ash in the dust conveying pipe per unit time. However, the traditional dust conveying method mainly relies on the compressed air at one end of the conveying pipe to drive. This traditional dust conveying pipe often causes dust blockage during use, and more seriously, the conveying capacity is limited, which seriously affects the boiler blending and makes the coal burning cost high, affecting the company's operating benefits. Moreover, the high flow rate of the dust in the dust conveying pipe causes large wear of the pipe. Therefore, it is urgent to provide a new type of dust conveying pipe to solve the problems of easy pipe blockage and serious wear of the dust conveying system during the dust conveying process. CONTENT OF THE UTILITY MODEL
[0004] The present application provides a pipe for preventing blockage of a power plant, which can not only reduce the risk of dust blockage in the dust conveying pipe, but also reduce the flow rate of the dust.
[0005] The above object of the present application is achieved by the following technical solution:
[0006] The application discloses a pipeline anti-blocking device for a power plant, which comprises a main air inlet pipe and a bin pump, wherein the air outlet end of the main air inlet pipe is connected with one end of a conveying pipe, the other end of the conveying pipe is connected with a designated ash bin, one side of the main air inlet pipe is further connected with an air inlet branch pipe, the air inlet branch pipe is connected with the air inlet of the bin pump, and the discharge port of the bin pump is connected with the conveying pipe.
[0007] A plurality of auxiliary ash conveying units are evenly arranged on one side of the conveying pipe along the length direction of the conveying pipe.
[0008] A plurality of auxiliary ash blocking removal units are further evenly arranged on the side of the conveying pipe opposite to the auxiliary ash conveying units along the length direction of the conveying pipe.
[0009] Further, the auxiliary ash blocking removal units are pneumatic vibrators which are fixedly installed outside the conveying pipe at corresponding positions through special clamps.
[0010] Further, the auxiliary ash conveying units are pilot automatic plug valves, and the main air outlet port of the pilot automatic plug valve is connected with the conveying pipe through a pressure guide pipe.
[0011] Further, a gas accompanying pipe is arranged on one side of the conveying pipe in parallel, compressed air prepared by an air compressor is conveyed in the gas accompanying pipe, a first air inlet pipe is connected between each of the pneumatic vibrators and the gas accompanying pipe in communication, and a second air inlet pipe is connected between the air inlets of the pilot automatic plug valves and the gas accompanying pipe in communication.
[0012] Further, the main air inlet pipe and the gas accompanying pipe are respectively connected with different air compressors.
[0013] Further, the distance between two adjacent pilot automatic plug valves on the conveying pipe is 5 m.
[0014] Further, the air pressure in the gas accompanying pipe is 0.3 Mpa.
[0015] According to the application, at least the following beneficial technical effects are achieved.
[0016] The application adds multiple auxiliary ash conveying units along the ash conveying pipe on the basis of the traditional silo pump fluidization system, each of which can blow in pressure gas at the corresponding ash conveying pipe, so that multi-point gas conveying can be carried out along the ash conveying pipe during the ash conveying process, and the compressed air power at the main air inlet pipe does not need to be too large, because the auxiliary ash conveying unit can timely dredge the dust at the blocked pipe and supplement the kinetic energy of the fluidized dust at the same time. Under the premise of reducing the compressed air power in the main air inlet pipe, the flow rate of the dust during the conveying process can be effectively reduced, so that the dust in the ash conveying pipe can flow to the designated ash silo in the ideal constant pressure and constant flow state as much as possible, and the corresponding dust wear on the pipeline can also be effectively reduced. Even if the pipe is blocked during the dust conveying process, the blocked dust can be flushed open by increasing the air pressure in the auxiliary ash conveying unit. The application further adds an auxiliary blockage removal unit on the ash conveying pipe, which can be further started when the auxiliary ash conveying unit has difficulty in processing the blocked position in the ash conveying pipe, and the auxiliary blockage removal unit is used to slightly vibrate the ash conveying pipe, thereby cooperating with the auxiliary ash conveying unit to effectively improve the blockage removal effect of the dust in the ash conveying pipe, so that the entire ash conveying system will be smoother during the dust conveying process. BRIEF DESCRIPTION OF DRAWINGS
[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.
[0018] Figure 1 is a schematic diagram of the overall structure of the present application;
[0019] Figure 2 is a schematic diagram of one of the pneumatic vibrators of the present application installed on the ash conveying pipe.
[0020] Reference signs: 1, main air inlet pipe; 2, silo pump; 3, ash conveying pipe; 4, air inlet branch pipe; 5, pneumatic vibrator; 6, pilot-operated automatic plug valve; 7, pressure guide pipe; 8, gas accompanying pipe; 9, first air inlet pipe; 10, second air inlet pipe. DETAILED DESCRIPTION
[0021] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor also belong to the scope of protection of the present application.
[0022] AsFigure 1 As shown, the anti-clogging pipeline for power plant disclosed by the present application comprises a main air inlet pipe 1 and a bin pump 2, the air outlet end of the main air inlet pipe 1 is connected with one end of a conveying pipe 3, the other end of the conveying pipe 3 is connected with a designated ash bin, one side of the main air inlet pipe 1 is further connected with an air inlet branch pipe 4, the air inlet branch pipe 4 is connected with the air inlet of the bin pump 2, and the discharge port of the bin pump 2 is connected with the conveying pipe 3 in communication;
[0023] A plurality of auxiliary ash conveying units are uniformly arranged along the length direction of one side of the conveying pipe 3, and the auxiliary ash conveying units can blow pressure gas into the conveying pipe 3 at the corresponding position during use;
[0024] A plurality of auxiliary clogging clearing units are uniformly arranged along the length direction of the other side of the conveying pipe 3 opposite to the plurality of auxiliary ash conveying units, and the auxiliary clogging clearing units can vibrate the conveying pipe 3 at the corresponding position during use.
[0025] In the above embodiments, the main air inlet pipe 1 of the application transmits air with pressure (also known as compressed air), one side of the main air inlet pipe 1 is connected with the air inlet of the bin pump 2 through the air inlet branch pipe 4, so that when the bin pump 2 is filled with dust, the valve on the air inlet branch pipe 4 is opened, part of the compressed air in the main air inlet pipe 1 can be introduced into the bin pump 2, and the dust particles in the bin pump 2 are converted into a fluidized state with small internal friction and adhesion. The fluidized dust is discharged from the discharge port of the bin pump 2 into the ash conveying pipe 3, and since one end of the main air inlet pipe 1 is connected with the ash conveying pipe 3, the main air inlet pipe 1 can use the compressed air in the pipeline to provide kinetic energy for the fluidized dust to drive the dust to flow along the ash conveying pipe 3 to the designated ash bin. Because the ash content of the coal burned in reality is relatively high, which can easily cause the ash conveying pipe to be blocked during the ash conveying process, and if it is not treated in time, the capacity of the ash conveying system will be limited. The application uniformly arranges multiple auxiliary ash conveying units along the ash conveying pipe 3, and the auxiliary ash conveying units can blow the gas with pressure into the ash conveying pipe 3 at the corresponding positions, so that when the ash conveying pipe 3 is blocked, the auxiliary ash conveying units near the blocked position can be started to quickly dredge the blocked dust, thereby ensuring that the dust in the ash conveying pipe 3 can be continuously conveyed. The air outlet size of each auxiliary ash conveying unit of the application can be freely adjusted, so that the air inlet can be adjusted according to the conveying site, that is, the air inlet is located where the pipe is blocked, and the air inlet is not located where the pipe is not blocked, so as to achieve the best conveying efficiency. In the traditional bin pump 2 fluidization system, in order to ensure that the dust can smoothly move along the ash conveying pipe 3 with a length of several hundred meters, the compressed air pressure in the main air inlet pipe 1 usually needs to be very high. On the one hand, the high-pressure compressed air in the main air inlet pipe 1 can ensure that the dust has enough kinetic energy to smoothly flow to the designated ash bin, and on the other hand, the high-speed flowing dust can reduce the risk of blockage in the ash conveying pipe 3, but in actual use, the effect is not good. Because the high-pressure and large-flow compressed air not only increases the flow rate of the dust, causing the ash conveying pipe 3 to be severely worn, but also the gas-dust ratio in the ash conveying pipe 3 is small during the ash conveying process, affecting the ash conveying efficiency. The multiple auxiliary ash conveying units uniformly arranged along the ash conveying pipe 3 of the application can freely adjust the air inlet pressure into the ash conveying pipe 3, and can dredge the blocked ash position in the ash conveying pipe 3 as needed during the ash conveying process, so even if the air inlet pressure in the main air inlet pipe 1 is reduced, there is no need to worry about the blockage of the dust in the ash conveying pipe 3. Therefore, in the actual ash conveying process, the air quantity for pneumatic ash conveying in the ash conveying pipe 3 can be effectively reduced, the conveying concentration is improved, the flow rate is reduced, and the gas-dust ratio of the dust flowing in the ash pipe is also correspondingly improved, realizing a relatively ideal constant pressure and constant flow state. In the case of the same amount of ash, the wear of the dust on the pipeline can be effectively reduced.
[0026] The application also uniformly arranges a plurality of auxiliary blockage cleaning units on the ash conveying pipe 3. When the pipe blocking dust in the ash conveying pipe 3 cannot be cleaned in time by the auxiliary ash conveying unit, the auxiliary blockage cleaning unit in the adjacent position can be opened to physically shake the ash conveying pipe 3 to shake the pipe blocking dust, so that the auxiliary ash conveying unit and the compressed air in the main air inlet pipe 1 can quickly cut into the gap of the dust, and the dust in the ash conveying pipe 3 can be quickly dredged. In this way, the risk of blockage of the ash conveying pipe 3 in the application can be further reduced.
[0027] Further, as shown in Figure 1 and Figure 2 , the auxiliary blockage cleaning unit is a pneumatic vibrator 5, which is fixedly installed on the outer side of the corresponding position of the ash conveying pipe 3 through a special clamp.
[0028] In the above embodiment, the auxiliary blockage cleaning unit of the application selects the pneumatic vibrator 5, because it has the advantages of simple structure, durability, low maintenance cost and the like when in use, and the pneumatic vibrator 5 is powered by compressed air, which can further utilize the on-site compressed air. In use, the pneumatic vibrator 5 can be bound on the ash conveying pipe 3 by a steel band, and a buffer device is arranged on the contact surface between the pneumatic vibrator 5 and the pipeline to protect the outer wall of the pipeline. Since the pneumatic vibrator 5 may cause vibration to the pipeline during use, in order to reduce the influence of the vibration of the ash conveying pipe 3 on the connection effect and the stability of the pipeline, a flange rubber joint can be used for connection at the connection between the pipe sections of the ash conveying pipe 3 or the connection between the pipe sections and the equipment, and the ash conveying pipe 3 is fixed by using an elastic support during installation. The pneumatic vibrator 5 used in the application is similar to the use mode of the pipeline special vibrator in the prior art, so during actual installation and use, the pipeline special vibrator in the prior art can be referred to, and the related content will not be described in detail.
[0029] Further, as shown in Figure 1 , the auxiliary ash conveying unit is a pilot-operated automatic plug valve 6, and the main air outlet port of the pilot-operated automatic plug valve 6 is connected with the ash conveying pipe 3 through a pressure guide pipe 7.
[0030] The pilot automatic plug valve 6 in the above embodiment is a commonly used device in the field of industry and fluid control, which can automatically adjust the output state of compressed air according to the pressure change in the pipeline when it is in use. For example, when the pressure in the ash conveying pipe 3 increases to exceed the preset value of the pilot automatic plug valve 6, the pilot automatic plug valve 6 will automatically open to guide the compressed air in the pipeline connected thereto into the ash conveying pipe 3 at the corresponding position. These gases can generate an impact force in the ash conveying pipe 3, thereby breaking the point that may cause the ash conveying pipe 3 to be blocked at the corresponding position, to ensure the smoothness of the ash conveying pipe 3 in use. In order to ensure that the compressed air discharged from the main air outlet port of the pilot automatic plug valve 6 can be smoothly blown to the blocked point and will not be affected by the compressed air flow in the main air inlet pipe 1 responsible for conveying dust, the pressure guide pipe 7 between the main air outlet port of the pilot automatic plug valve 6 and the ash conveying pipe 3 is arranged obliquely, and the horizontal distance from the connection point of the pressure guide pipe 7 and the ash conveying pipe 3 to the designated ash bin is less than the horizontal distance from the connection point of the pressure guide pipe 7 and the main air outlet port of the pilot automatic plug valve 6 to the designated ash bin.
[0031] Further, as shown in Figure 1 one side of the ash conveying pipe 3 is parallel to a gas accompanying pipeline 8, the gas accompanying pipeline 8 conveys compressed air prepared by an air compressor, and the plurality of pneumatic vibrators 5 are respectively connected to the gas accompanying pipeline 8 through a first air inlet pipe 9, and the air inlets of the plurality of pilot automatic plug valves 6 are respectively connected to the gas accompanying pipeline 8 through a second air inlet pipe 10.
[0032] In the above embodiment, the pilot automatic plug valve 6 and the pneumatic vibrator 5 of the present application both need to rely on compressed air, and they are uniformly arranged along the length direction of the ash conveying pipe 3. Therefore, the present application parallelly lays a gas accompanying pipeline 8 on one side of the ash conveying pipe 3, which is used to stably supply compressed air to the plurality of pilot automatic plug valves 6 and the plurality of pneumatic vibrators 5. The plurality of pilot automatic plug valves 6 of the present application are all supplied with air by the gas accompanying pipeline 8. When the pilot automatic plug valve 6 is used to blow air to the blocked position of the ash conveying pipe 3 for dredging, for a single blocked point in the ash conveying pipe 3, the blocked position can be broken by increasing the air intake of the auxiliary ash conveying unit, but if multiple blocked points appear in the ash conveying pipe 3 at the same time, the gas accompanying pipeline 8 needs to increase the air supply effect to the pilot automatic plug valves 6 at the multiple blocked points. Since the pilot automatic plug valve 6 needs to continuously consume a large amount of compressed air to clear the blocked points in the pipeline, this will cause the compressed air in the gas accompanying pipeline 8 to be unstable. Moreover, simply relying on gas to impact the blocked points in the pipeline has the problems of single clearing method and poor clearing effect. For example, some hardened ash is simply swept by a blower, and the clearing effect is very limited.
[0033] The pneumatic vibrator 5 added on the dust conveying pipe 3 relative to the auxiliary dust conveying unit side adopts a physical vibration to disperse the caked material in working, which is different from blowing with gas, and can make the blocked dust scattered and spaced to form gaps, so that the compressed air generated by the pilot automatic plug valve 6 can quickly drill into the dust to break the blocked dust. Compared with the way of relying on the pilot automatic plug valve 6 to clear the blockage, the efficiency is obviously higher. Moreover, the pneumatic vibrator 5 can work intermittently and does not need to be supplied with compressed air continuously. For example, when the pilot automatic plug valve 6 cannot clear the blockage by relying on the conventional air pressure, part of the compressed air in the air accompanying pipeline 8 can be introduced into the pneumatic vibrator 5. The pneumatic vibrator 5 can vibrate for about 10 seconds to shake the blocked dust, and then the pilot automatic plug valve 6 can easily clear the blocked position without increasing the air inlet. Compared with the way of blowing and clearing the blockage by the pilot automatic plug valve 6 alone, the air consumption can be effectively reduced. The long-term inflow of high-pressure gas in the pilot automatic plug valve 6 can affect the service life of the internal parts, and the corresponding cost may increase in subsequent maintenance. Compared with the pilot automatic plug valve 6, the internal structure of the pneumatic vibrator 5 is much simpler. The increased air pressure of the pilot automatic plug valve 6 is divided into the pneumatic vibrator 5, which can reduce the risk of damage to the internal parts of the pilot automatic plug valve 6 due to the long-term flow of high-pressure air, and the corresponding cost of subsequent maintenance can be effectively controlled.
[0034] Further, the main air inlet pipe 1 and the air accompanying pipeline 8 are respectively connected with different air compressors.
[0035] In the above embodiment, the air accompanying pipeline 8 responsible for supplying compressed air to the pilot automatic plug valve 6 and the pneumatic vibrator 5 and the main air inlet pipe 1 responsible for conveying dust in the dust conveying pipe 3 are respectively connected with different air compressors, so that the interference of their air pressure stability when used at the same time can be reduced.
[0036] Further, the distance between two adjacent pilot automatic plug valves 6 on the dust conveying pipe 3 is 5 meters.
[0037] In the above embodiment, the pilot automatic plug valves 6 are evenly arranged on the dust conveying pipe 3 with the above interval, so that the pilot automatic plug valves 6 can cover all possible blocked areas on the dust conveying pipe 3 as much as possible when used.
[0038] Further, the air pressure in the air accompanying pipeline 8 is 0.3 Mpa.
[0039] In the above embodiment, in actual production, for a common dust conveying pipe 3 with a length of about 500 meters and a diameter of about 14 mm, the pressure of the air source conveyed in the air accompanying pipeline 8 is 0.3 Mpa, which can meet the basic air requirement for normal operation of the conveying system.
[0040] The implementation principle of the embodiment is that when the compressed air in the main air inlet pipe 1 is used to transport the fluidized dust falling from the ash bin pump 2 along the ash conveying pipe 3, once a pipe blockage occurs in the ash conveying pipe 3, the pilot automatic plug valve 6 at the corresponding position of the ash conveying pipe 3 will be started first, and part of the compressed air in the air accompanying pipe 8 is blown into the ash conveying pipe 3 through the pressure guide pipe 7, and the compressed air blown out is used to clean the blocked dust. When the blocked and hardened dust in the ash conveying pipe 3 is relatively stubborn, the pneumatic vibrator 5 at the corresponding position of the ash conveying pipe 3 will be opened, which can use the compressed air obtained from the air accompanying pipe 8 to drive the vibration of the ash conveying pipe 3 at the corresponding position, and the particles of the hardened dust in the ash conveying pipe 3 will resonate in the process of vibration of the ash conveying pipe 3, so that the gap between the hardened dust and the ash conveying pipe 3 occurs occasionally, which can facilitate the rapid invasion of the compressed air generated by the pilot automatic plug valve 6, so that the blocked dust can be quickly dissolved. Compared with the prior art, the pneumatic vibrator 5 and the pilot automatic plug valve 6 are cooperated to greatly reduce the probability of pipe blockage of the ash conveying pipe 3 during the conveying of dust, so that the amount of air in the main air inlet pipe 1 can be reduced, and the dust can flow to the designated ash bin along the ash conveying pipe 3. Since the ash conveying flow rate is reduced and the ash conveying capacity is improved, the problem of serious wear of the ash conveying pipe 3 can also be effectively improved.
[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can modify the technical solutions described in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.
Claims
1. An anti-clogging duct for use in a power plant, characterized by: It comprises a main air inlet pipe (1) and a bin pump (2), the air outlet end of the main air inlet pipe (1) is connected with one end of a conveying pipe (3), the other end of the conveying pipe (3) is connected with a designated ash bin, one side of the main air inlet pipe (1) is also connected with an air inlet branch pipe (4), the air inlet branch pipe (4) is connected with the air inlet of the bin pump (2), the discharge port of the bin pump (2) is connected with the conveying pipe (3); A plurality of auxiliary ash conveying units are evenly arranged on one side of the conveying pipe (3) along the length direction, the auxiliary ash conveying units can blow pressure gas into the conveying pipe (3) at the corresponding position during use; A plurality of auxiliary blockage removing units are also evenly arranged on the side opposite to the auxiliary ash conveying units along the length direction of the conveying pipe (3), the auxiliary blockage removing units can vibrate the conveying pipe (3) at the corresponding position during use.
2. The anti-jamming duct for power plants according to claim 1, characterized in that: The auxiliary blockage removing unit is a pneumatic vibrator (5), the pneumatic vibrator (5) is fixedly installed on the outside of the conveying pipe (3) at the corresponding position through a special clamp.
3. The anti-jamming duct for power plants according to claim 2, characterized in that: The auxiliary ash conveying unit is a pilot automatic plug valve (6), the main air outlet port of the pilot automatic plug valve (6) is connected with the conveying pipe (3) through a pressure guide pipe (7).
4. The anti-jamming duct for power plants according to claim 3, characterized in that: One side of the conveying pipe (3) is parallelly laid with a gas accompanying pipe (8), the gas accompanying pipe (8) is used for conveying compressed air prepared by an air compressor, a plurality of pneumatic vibrators (5) are respectively connected with the gas accompanying pipe (8) through a first air inlet pipe (9), and the air inlets of a plurality of pilot automatic plug valves (6) are respectively connected with the gas accompanying pipe (8) through a second air inlet pipe (10).
5. The anti-jamming duct for power plants according to claim 4, characterized in that: The main air inlet pipe (1) and the gas accompanying pipe (8) are respectively connected with different air compressors.
6. Anti-clogging duct for power plants according to any of the claims from 3 to 5, characterized in that: The distance between two adjacent pilot automatic plug valves (6) on the conveying pipe (3) is 5m.
7. The anti-jamming duct for power plants according to claim 4, characterized in that: The air pressure in the gas accompanying pipe (8) is 0.3Mpa.